Well, if you are already in a relationship then it should be pretty easy. Invite him round to watch a movie, get close on the couch and just start with kissing and touching, and then as it gets more passionate let things take its course...
That's how it happened the first time I made out with a girl, although I was planning to kiss her for ages. But it didn't go much further anyway because we were a bit inexperienced and nervous hahaa