Shamanic work I feel is something you have to practice rather than read about. Although to be well read is not a deficit! It's about various experiential consciousness that would bring about a message and/or reconsider a point of view. Many groups do this through a pathworking type meditations among other practices. It also is a path of the Priestess/Priest to help facilitate sacred space that will bring about those messages to the seeker. I would say most of us on the magical path already practice some form of shamanistic work.
Finding a reliable group to work with is the best way to learn.
While it can be daunting to find "honest" teachers, following ones heart should lead you in the right direction. If ever someone claims to know it all or want cash to join, the gut will warn you.
The link Lady provided actually listed the link that Spirit provided, but after reading about the Foundation for Shamanic study, they seemed pretty legit and on the up and up. They don't provide study about any tradition that hasn't come to them first. They are about protecting these practices from being lost. I was also sad to find a man listed in there whom I have circled with many times who couldn't be a more real and authentic if he had been born 100% full blood whatever. That's the problem, there are too many cultural people that have fallen many generations from their roots and still feel it in their blood, yet they are orphaned and there is no one to teach them. Of course anyone who is selling themselves (or items) for profit as an Indigenous healers is what they are wanting to stop, I get that, but people have a right to explore practices that feels like a blood memory. Since they have no teacher it's a re-creation. What's an orphaned Native to do? They go to the internet.
It would behoove many a tribe to open their traditions before they are lost to time.
This not meant to irk anyone but being an orphaned Native (Lenni Lenape) myself, I rather resent some