You need a major influence with Bao-dur in order for him to
become a jedi. If you are a light or neutral Jedi, Bao-dur
will start a conversation with "You know General, having you around has an effect on me." (or something like that)
"I haven't noticed it before. maybe I was too occupied then." Say nothing to end the conversation. Then he says,(approximately)"There is an anger inside me. It is still there, but I feel it drifting away." At this point you can train him to become a Jedi guardian. If his alignment is in the lightside zone, he will agree.