In Spiritualism, it is believed that the mind of a person continues after death in the form of a discarnate spirit. These spirits remain on a plane overlapping ours after death, called the Veil, also known as the Valley by Wendell. The Veil is a border world between our physical plane and the spirit world. Spirits are able to telepathically whisper to us through the Veil, but ordinary consciousness drowns their voices out.
In order to hear the voices of the spirits, one must metaphorically press their ear against the Veil while tuning out their material sensations and loud thoughts. This is done through the "death trance." The purpose of this trance is to bring the mind close to what it would experience in death. Communication is a two-way street; while the spirits of the dead need to approach the living by entering the Veil, the living need to approach death to hear them. This also provides a Spiritualist account of "death bed phenomena;" the closer one is to death, the easier it is to commune with the dead. While necromancers used to poison themselves with dissociatives and psychedelics to facilitate this, Franz Mesmer discovered that a much easier way to produce these experiences is through trance.
The death trance is quite simple in theory. The goal is to quiet the mind as completely as possible, and then carefully focus on "listening" or "receiving" from across the barrier of death. This can take laborious practice to accomplish on command. I recommend a practice regimen of about 10 to 15 minutes a day, but no longer. Close your eyes and try to find somewhere quiet to sit. This process is easier when you are comfortable, as discomfort and pain can be distracting, which is precisely what you are trying to tune out. It is still possible if you experience pain and noise, it is simply more tedious to constantly have to refocus. Eventually, you can become skilled enough that you can accomplish this silence of mind on command regardless of circumstances, although this is not normally necessary.
Once you reach this emptiness of consciousness, it should feel mildly effortful to maintain. Along with sitting, this will keep you awake, which is important if you want to remember the communications. When it comes to listening, you want to listen to what lies outside of you through the darkness that sits underneath your consciousness. This is hard to describe in words, but you want to shift your focus away from your mind and through the nothingness itself in order to listen to it attentively.
At this stage, you may begin to feel a presence, and this is what you are looking for. From this presence, you may receive "impressions" or "feelings" through what can best be described as a kind of sixth sense. If you are incredibly skilled, you may even hear voices in a mental voice that is unmistakably not yours. It can be quite jarring to hear a different mental voice in your head, and it often makes you aware that your memories of other people's voices are simply your mental voice doing an impression of them. Do not worry if you cannot hear any voices; feeling a presence is all you need to conduct a seance.
I recommend that you hold a seance once a week. As you conduct these seances, you will find yourself no longer needing to rely on physical mediums such as pendulums or spirit boards. You will simply "know" (clairsentience) or "hear" (clairaudience) what a spirit is going to say long before your planchette or pendulum will finish moving. Pay special attention to when the spirits tell you something that you could not have otherwise known, because this is a sign that you are holding your trance with impeccable form. Remember to verify what the spirits have told you. If their information is accurate, then during your next seance, try to recreate the precise state of mind you were in when you received that information; it will be some form of refined death trance.
When you listen through the Veil, spirits in your area can see that you are dipping into their plane. This is often harmless, although it can cause them to talk over each other in an effort to finally be heard by someone on this side. Sometimes, you attract the attention of spirits with more malevolent intentions. This can result in anything from simply feeling a malevolent presence to experiencing hostile poltergeist activity. For that reason, seances are built with several safety protocols in mind.
While it is not strictly necessary, I highly recommend reading "The Necromantic Ritual Book" and taking a look at the "Basic Magic" exercises on Real Magick before conducting a seance. "The Necromantic Ritual Book" will help you focus on Death himself for the sake of communication, and "Basic Magic" will help you acclimate to the general energy work that will help protect your space and call forward the proper spirits. I will give a brief summary of the relevant information here, for your convenience.
In general, in order to latch on to the specific energy of anything, whether it is Death, a particular spirit, or a classical element, you want to surround yourself with objects and symbols that are saturated in these energies. For example, for Death, you could go to a graveyard and meditate on a skull next to a dead bird. When you are well-practiced with the death trance, you can choose to listen "through" these objects and symbols in order to listen "to" the underlying energy they all share. When you are surrounded by these objects and symbols, you should be able to "tune in" to the energy that they have in common, which will contrast with unrelated symbols and objects. Focus that sixth sense at them.
This is called "channeling." It can be tricky to achieve at first, but this is a fundamental skill of magick in general that is absolutely required for practices as complex as necromancy. While you can technically stop learning once you master the death trance and simply work with local spirits in your area, you will not be able to protect yourself, verify the identity of spirits via their energy signatures, or summon particular spirits for conversation. In the method that I describe here, the only specific spirit you need to learn to channel is Death, because Death will take care of the rest of the process for you. Wendell kept an extensive album of representations of Death on her site, and I have a carefully cultivated list of aspects of Death for you to learn about and focus on in ritual, too.
Before you summon spirits, you should learn to banish and exorcise them. In Western necromancy, many of the authors of goetic texts were exorcists employed by the Catholic Church who learned to invert the principles behind exorcism. This is an extremely important safety precaution.
A traditional method would be to perform the Lesser Ritual of the Pentagram invoking earth, then conduct one of a few exorcist rites recognized by the Church, then perform a Lesser Ritual of the Pentagram banishing earth. Frankincense, myrrh, and Palo Santo are used for incense, and holy water aspersions are done at the corners of the space. Earth is used because it is tied to "worldly" or chthonic spirits, which includes spirits of the dead as well as spirits associated with the underworld in general, such as fallen angels.
Instead of this, I recommend a method that relies entirely on Death. To pray effectively, you must first channel Death until you feel both his presence and his attention; you need to feel that he is listening to you. Pray to him, asking him to reveal the spirits around you or let you know if there are no spirits around you. If there are spirits around you and you are listening properly, you will feel their presence wherever they are in the space around you.
Focus on one of these spirits, then pray to Death to remove them from the location. After this prayer, invoke Death by saying, "Through the power of Death granted to me, I banish you from this space." As you say this, "push" the spirit out of the space with your mind; it takes practice.
Trickier spirits will try to hide or cast illusions to make it seem like you have successfully cast them out, when in reality they have not left. The workaround for this is quite simple; all you need to do is repeat these three prayers with the proper focus. Eventually, they will not be present when you pray to Death to reveal them to you. It does no harm to pray for the spirits to be revealed a few more times after this happens, just to be thorough.
I recommend against conducting your first few seances in or near anywhere you frequent, if it can be helped. The best location for your first seance is a graveyard when nobody else is around. Failing that, try to at least conduct the seance in your backyard, a room you do not enter frequently, or an antechamber. If none of these are available to you, you can conduct a seance in your own home, but be sure to take the safety precautions seriously.
The first safety precaution is to take your divination medium and perform a banishing ritual on it. Divination mediums that work for seances include spirit boards, divination cards such as tarot, pendulums, black mirrors, crystal balls, and cast-iron cauldrons. Each of these mediums work a little differently, and you will want to conduct a banishing on them both before and after the seance.
Secondly, you are going to want to bless your divination medium. Channel and pray to Death, asking him to imbue the selected medium with his essence. It helps to focus on a version of Death that also feels pure and holy. If he accepts, you should feel him acknowledge your request and move from you into the object. Visualize his movement as a shadow leaving you and becoming aborbed by the divination medium. As the medium absorbs the shadow, visualize all of the crusty, dirty energy being washed out of it.
The third safety precaution is to do these same banishings and blessings on the space you are conducting the seance in. If you are in an open area, such as a graveyard or a backyard, then mark this space with a blanket or a rope tied in a circle. Bless this blanket or rope, too, before using it to set a sacred space. If you are in a room, all you need to do is banish and bless the room itself.
Now, you can begin your seance. Channel and pray to Death, asking him to guard you and your sacred space during the seance. Then, ask Death to open a window into the Veil, and ask him to safely relay the messages from the other side to you. Have your divination medium ready.
Enter the death trance and use your divination medium. Try to maintain the death trance as much as you can while you use your divination medium, even though you cannot hold it perfectly. If there is a particular spirit you wish to talk to, channel and pray to Death in order to request them before you use your divination medium. They may be unwilling or unable to talk to you, which Death should inform you of.
When you are finished, request that all spirits leave and give them a moment to do so before saying "goodbye." Then, perform a banishing in the space and on your divination medium. Push any spirits revealed by the banishing through the open window back into the Veil. Then, channel and pray to Death, asking him to close the window.
Do another round of banishings, followed by another round of blessings. Channel and pray to Death, thanking him for his help. Ask Death to guard you when you leave the sacred space and give him license to depart at his leisure. Now, you can clean up and leave the space.
Spirits are not omniscient and cannot necessarily see the future, so keep this in mind when interacting with them. Powerful diviners of the past may be useful to contact if you are looking for fortune telling. Some spirits can give you life advice, which is perhaps the most reliably useful application for necromancy. Many spirits forget what they knew in life, but they also learn new information from other spirits they talk to in the underworld.
Spirits can be mistaken in death as easily as they could be mistaken in life. If you receive information that is incorrect, it is possible you received a genuine communication from a lying or mistaken spirit.
Spirits primarily interact with the world through subconscious telepathic communication. Spirits are also capable of performing their own rituals and spells on your behalf. They are not omnipotent and, in fact, are often quite limited.
You may wish to give your spirit more power by grounding it, either with its corpse or grave soil taken from where its body was buried. You can use animal bones for this, although it is not as effective. Spirits tend to enjoy offerings of blood and tobacco smoke, and you can pour libations for them into grave soil. Be careful with who you offer your blood to, as it can be used against you as a powerful taglock; fresh animal blood is often preferred.
Different spirits have different levels of power. Spirits from specific traditions of ancestral worship will often refuse to work with you or even spite you if you do not approach them in the traditional way. These spirits also tend to be quite powerful, because they are constantly receiving offerings and being grounded by their worshipers. Do not use the methods I outlined above for interacting with spirits from Vodoun, Quimbanda, or Palo, because they demand very specific approaches as a sign of respect, as well as for other reasons. Your own ancestral spirits may be quite powerful if you maintain a memorial for people you knew in life, especially people you are related to by blood.
Some of the traditional Hermetic trappings can be used to increase the power of a spirit for the sake of a specific working. For example, angels and demons can "clear the way" for a spirit to bring about a certain manifestation, and they can be used effectively against more powerful spirits. Saturn and Pluto are tied to chthonic energy, so you can use this for ritual implements, timing, Saturnian sigils, Saturnian pentacles and spirits, etc. There are also a number of summoning circles, binding triangles, ritual instruments, and magic seals that help to bring a spirit closer to the physical plane, allowing it to work quicker and more effectively.
Be aware that deals with these spirits often come at a cost beyond the initial offering. You can ask Death to reveal these hidden costs to you. These hidden costs are normally caused by the universe balancing out the effects of your request and/or by the spirit taking advantage of ambiguity in your request to benefit themselves at your expense. The cost is normally felt by the means in which the result manifests. For example, you could ask for a life-saving amount of money, only for you to receive it as inheritance from the death of one of your parents.
You can ask Death to help you find the right spirit for a task and to help you form your request in a way that minimizes these hidden costs. However, these hidden costs are always going to exist in some form, although the costs are often subjectively "worth it" or will even seem subjectively irrelevant to you. Between the hard limits on the power and abilities of the spirits themselves and these hidden costs, ghost magic can only do so much; this is not a genie in a bottle.