Yoda tells Obi Won Kenobi that he was going to teach him how to do the force-ghost at the end of Star Wars III: Revenge of the Sith. Qui Gon Jin came back from the etherworld & began teaching Yoda. But then Yoda & Ben go their separate ways. So, how was Yoda able to teach Ben if they were both on different planets? Ben was on the sand planet, Tatooine, and Yoda was on Dagabah. Yoda says to Ben Kenobi, "More teaching I have." So, did they have walky talkies? Or did Qui Gon Jin just go teach Ben himself?
Yoda could have just told him the basics of how to do it and then left him to practice it on his own during his exile.