[Insert standard disclaimer here] ^_^

It depends on just how stuck the paper is. There 
are several ways, but here are two common ways 
someone might do it:

1) Freezer method- this has varying success, and
   I've found it works best with partially or 
   lightly stuck cels...

2) Water method- the guaranteed way to get stuck 
   paper off a cel (lightly dampen, don't soak),
   although you *will* sacrifice whatever paper 
   is stuck to the cel. Some people recommend 
   distilled water to prevent mineral deposit from
   normal tap water and the like (no, not bottled
   drinking water ^_^''), though I use tap water 
   myself ... 

Always - be *very* careful when doing a paper 
removal. This is all amateur experience, of 
course, if you are really wary of handling a cel,
you can take it to an animation gallery/store 
that may do it for you.

^_^