Usually, I'll just do it naturally, as if I had no control. If I felt I needed to poop while in bed, I'd just let it out. It's more immersive that way, y'know. It's just my personal favourite way of messing myself, feeling like a big baby!
Aside from that, I do enjoy squatting down and doing it sometimes, and also sitting on the potty while wearing diapers. Especially that last one, it feels like you're failing potty training, obviously that's what makes it so good.