These are commonly used and tried shamanic techniques that shamans use the interact with spirits . These techniques are part of animism .
Negotiating with spirits
The most common way of interacting with spirits is by trancing and then creating a deal with a spirit. This might involve giving the spirit divine energy directly in the form of worship, doing something for a spirit in the real world (usually following some sort of bane), attacking some other spirit, letting the spirit posses you for a while (see below) or providing some form of information. In exchange the spirit can influence the material realm in ways the shaman wishes. For instance; a spirit might hide a shaman from enemies or produce magical fire to burn his enemies during combat. What a spirit can do for you depends highly on the nature and power of the individual spirit.
Spirit banes
Although all spirits want power and security because of their nature they often have rather weird aliens ways of gaining and securing their power. Most mortals don't know and might not want to know why a spirit has such strange needs as "every third baby in the village needs only 1 tumb." or "No merfolk, nymph nor slemmering may ever step in between these two trees." but when the shaman says the spirits demand it... there is probably hell to pay if the instruction isn't followed to the letter. Shamans themselves are often bound and propelled by many strange and limiting banes. Some are as simple and understandable as "You must thank the river for every meal with fish in it." or "Never should you wear anything made of wood" but other can be as arcane as "two nights before every full moon you must make a different person ingest something yellow drenched in pig blood" or "You are never allowed to defend yourself when somebody attacks. Either strike first or flee.". These banes are payment for the services the spirits provide for the shaman.
This often gives the shamans a fearful reputation of being insane, listening to the strange beings that only they can hear, and only when they has ingested enough hallucinogenics. Many shamans have been accused of making up banes just to have an excuse for their behaviour or to convince people that a certain thing should be done. Other shamans listened to banes of spirits that have never even existed but that they once witnessed in a fever dream or met in a bad acid trip... some of those spirits become real by proxy after a while.
Lastly... some shamans truly are insane and never speak to spirits at all.
Directly attacking and consuming spirits
A shaman can enter the spirit realm and consume a spirit if the shaman is strong enough. In the past this technique has been tried to see if this results in the same form of power transfer that happens when spirits do this. The conclusion was that there was no direct power transfer. Although this had an effect on the soul, the subject did not receive any powers comparable to those of spirits. A further disadvantage of this technique was that the subjects personality changed.
Encapsulating the spirit to later transfer it to a different spirit has also been attempted. It proved to be very difficult to keep the spirit and your own soul separate, which resulted in personality changes, especially over longer periods of time.
A more direct method that is commonly used by shamans is hunting spirits with the help of allied spirits. These allied spirits can consume the prey at the end of the hunt. This indirectly increases the power of the shaman by strengthening its allies.
Becoming a spirit
Especially non-shaman mortals but also some shamans, see spirits as higher beings than themselves. This has led to attempts to become a spirit - to increase power, knowledge, become immortal or other reasons.
After many tries the most simple method was found. The shaman enters the spirit world and breaks the bond it holds with his body. In this state, the shaman is very weak and will most likely be consumed by other spirits. At this point the shaman is a spirit.
With lots of preparation a shaman can create a very strong spirit body before breaking the bond. This can be done by spending a lot of time in the spirit world, usually years on end. Most of this time is spent hunting other spirits and consuming them, as preparation and training on how to live as a spirit. As mentioned earlier, this changes the shaman's personality and causing many shamans to abandon this goal.
Voluntary Possession versus Channelling
Spirits can aid shamans in a lot of situations. If arrangements are made in advance, the spirit can be paid to strengthen or regulate some parts of the mortal body. Examples include lending a mortal physical strength, mental capacity or social confidence. More esoteric examples are giving a mortal the ability to understand certain languages, see in the dark or become highly proficient with a certain weapon. This is called channelling.
A spirit can alternatively - usually without payment - take this process a step further and fully posses the body of the mortal. The difference between possession and channelling is that during possession, the spirit is in complete control. Although a mortal can potentially wrestle back control, most of the time the possession will last until the spirit leaves the body - either voluntarily or by an outside force. In contrast, during a channelling certain personality traits of the spirit can manifest, but the mortal is in control and can at any time stop the process.
For the spirit, the difference between the two techniques is that during possession the spirit can experience the sensations of the body. This gives the spirit immense pleasure since they normally don't have access to these senses. It also gives the spirit access to energy resources it normally doesn't have. Additionally, it can influence the material realm in ways it has trouble doing from the spirit realm, by making the body do things the mortal would never do.
Shamanic afterlife
During negotiations with spirits a shaman can give a part of his soul as payment, usually only after the death of the shaman. This deal may take such a form that all spirits the shaman has a connection with, will receive their fair share of the soul after death. In this situation the entire soul is promised to the spirits. Spirits often demand such a deal as part of the payment for staying with the shaman for a longer term. Many shamans are promised to their spirits In this way and see it as a full fledged afterlife. In theory the entire soul can be promised to a single spirit. Some shamans prefer this, because they theorize that a larger part of their personality and consciousness will remain.
If a shaman is baptised at any point in their lives, the soul is promised to a deity. This can be circumvented by entering the spirit world just before death. This prevents the shaman's soul from disappearing or being consumed by spirits that are not bound to the shaman.
If the shaman doesn't enter the spirit world before the death - for example, if the shaman dies in combat - the soul will disappear and may not return to the spirit world. Some stories circulate among shamans about angry spirits that did not get to consume the soul of their recently deceased shaman, only to ecstatically report at a later date, that the soul has arrived anyway.