That's hard enough even when one is starting from a clean slate. Its much harder when one must first unlearn something. And very very hard if one has first paid good $$$$ for some guru from a respected organization and studied hard their doctrine.
"Only the self taught can be called learned ones, the others are all trained ones". There is some truth in this.
Teaching large groups requires doctrine and a program, since individual attention is too costly. Thus the program must be up to snuff, and not detrimental to useful learning.
Commercial viability sets cruel contingencies. Consulting on an individual bases is very expensive, but obviously would be best for the student who can afford it. And who is lucky enough to get a good teacher.
Lurking around on this board is probably best !