Hey kind of new to this whole wam thing but have always fantasized about being completely covered in peanut butter and jelly and getting super messy with it. Any ideas on the best way to do this?

Yes, go and buy some.

That would be too easy

There are several aproaches depending on where you are, ambient temperatures, privacy, etc.

Assumning wanting to do it indoors, and having complete privacy, then wrap a bed completely in several layers of plastic sheeting - use the heavier gague stuff builders use for temporary emergency windows, as the "dust sheets" they sell in DIY stores are useless and split as soon as you breathe on them much less writhe about on top of them all sticky.

Make a trail of towels from the bed to the nearest shower, don't leave any gaps and depending on how precious you are about carpets, etc, you might want to lay newspapers under the towels.

Obtain large quantities of peanut butter and jelly (jam in UK context), and the outfit you want to wear while doing it (if any).

Jars within easy reach, probably on a hostess trolley, beside the bed. Protect the trolley with newspaper and/or towels too, less to clean up afterwards.

Tip 1: Open all the jars before starting. Trying to open a jar of jam with slippery hands is no fun.

Tip 2: Flim your session. That way you'll have a reason to keep going and get totally covered even if you actually achieve a result early on, and you can enjoy the experience multiple times afterwards on video. I always found having the footage to look at afterwards made the amount of work involved in setting a session up more worth while.

Have fun!