Like this post?
Imagine you have a vagina, and you are to finger yourself(which, in the case of us males, would be to stimulate our prostate). You'd really have to try all positions you can think of and figure out which one you're most comfortable with. But I'd imagine you'd also want to be able to stroke your penis while you masturbate anally...
The way I do that is to lie on my back with a towel spread beside you, and do what I just explained above. Then, as you near the point of no return, you turn to the side to where the towel is, and ejaculate on it so as to keep the bed from getting soiled with semen.
Hopefully you'll find this helpful at least somewhat... lol
All the best.
There's Peter Pan in each of us.