The best thing to do is try to pretend you're a baby or toddler or whatever your fantasy is, then act accordingly. Get so into the role that you think it's real. I've done that without even trying to when cross-playing in online RPGs (playing and acting as the opposite gender online), to the point where I was uncomfortable sitting at my computer shirtless, because I felt that girls shouldn't be topless like that. I'm not transgendered or a sissy, but it was a very fascinating experience. For a short time, my subconscious was convinced that I was female. If you can achieve the same thing with your AB fantasy, you'll have an incredible time.