Practice
Suppose you have chosen the previous life before the current one, and are ready to plunge into a light trance. Now take a comfortable position, sitting or lying (your back should have support). If it's noisy around, put on headphones with meditative music, but do not turn it on too loudly. You may use a recording of a shamanic tambourine.
You should sit or lie in a comfortable position. Shamans use the «non–doing» during practice; that is, they put on some unusual clothes to show the spirit that they are in an altered state of consciousness. For the first session, you can also put, for example, a bandana on your head. Any unusual action helps the attention to concentrate, which generally facilitates the practice. But you can dispense with it. It's also good to use crystals that you can hold in your hand while traveling into the past.
Entering the shaman's cave
Close your eyes and listen to the music or the silence around you. Stop the inner monologue. At this moment, you should neither think about everyday things nor mentally comment on anything that is happening. Now imagine a cave in the mountain. It may be a real cave that you've seen before, or one made-up by yourself; this does not matter. The cave is a secret entrance to your subconscious. If you see only one cave, then you have managed to open one entrance so far, but it may turn out that there will be several caves in front of you, each of which leads to a past life. This implies that your subconscious mind offers you a choice.
Look around: What do you see? What does the place where your cave is located look like? Look up at the sky, and then look down. What is around? Is it day or night outside? It may be a forest, a desert, or even a city around you. Afterwards, you will always use this entry point. You should be cozy and comfortable In this place, and you also should understand that it is absolutely safe here, only you are in this space. No outsider may appear here without your consent. Shamans in their practices call for helpers, spirits of trees or animals with whom they got acquainted before, but this is not necessary to view past lives. Your higher Self, your spirit, is quite capable of independently penetrating into the secrets of previous births.