When I first started messing, I had a tough time with it. I've learned if I relax enough, I can do it rather easily. Although on occasion I'll help it. That's only because I want the feel of a dirty diaper, just like a baby.
I find messing easier than wetting. Unless I'm squatting or standing, my bladder sphincter squeezes itself shut and won't let anything out, no matter how bad I have to go. My bowels do not have that problem.
I'm incontinent so whatever happens happens. But I think it takes time to mess or wet because your body is not used to going in a diaper. Just give it some time.
I can only go when I'm on the potty. I can't seem to go anywhere else because my bladder is trained to only use the potty. :c That's one of the reasons I stopped wearing my Depend pull-up underwear unless that little bothersome woman thing comes.