One becomes a shaman by one of three methods:
a) Hereditary transmission;
b) Spontaneous selection or "call" or "election";
c) personal choice and quest. (This latter method is less frequent and traditionally such a shaman is considered less powerful than one selected by one of the two preceding methods.)
The shaman is not recognized as legitimate without having undergone two types of training:
1) Ecstatic (dreams, trances, etc.)
2) Traditional ("shamanic techniques, names and functions of spirits, mythology and genealogy of the clan, secret language, etc.) The two-fold course of instruction, given by the spirits and the old master shamans is equivalent to an initiation." (Mircea Eliade, The Encyclopedia of Religion, v. 13 , p. 202; Mcmillian, N.Y., 1987.) It is also possible for the entire process to take place in the dream state or in ecstatic experience. Thus, there is more to becoming a shaman than a single experience. It requires training, perseverance and service.
Suggested free e-books to read:
Pt Shriram Sharma Acharya - The Eternity Of Sounds And The Science Of MantrasWilliam Godwin - The Lives Of The Necromancers
Ann Groa Sheffield - Groa List Of Recommended Heathen Reading
Nick Farrell - Notes On Geomancy