At the end of Return of the Jedi (movie #6) there is a scene on Endor where the victorious Luke Skywalker sees the Force 'ghosts' of Obiwan Kenobi, Yoda...and Anakin Skywalker.
I don't recall it ever being mentioned in the movies, but the novels had it that Qui-Gonn Jinn studied the 'Whills' teachings and that these teachings were what allowed him to become a 'ghost'. He then supposedly taught the technique to Obiwan and then sometime taught Yoda?
If this is actual canon, then I'm puzzled by how Anakin Skywalker learned how to become a Force ghost. He would have been Darth Vader since the end of movie #3 (before Obiwan learned) and I can't see either he or Yoda teaching a Sith Lord the skill.
Was it ever mentioned in the books or revised movies how Anakin made the Force Ghost team (as it were)? Or was it implied that he was just so powerful in the Force that he automatically knew how to do it?