Fixing a patch in a wing
 

Try this: cut the piece almost all the way to each rib. If there are spars a little ways ahead and behind the patch cut the hole to those. Fold down any overlap that's left over and stick it down. You do NOT need CA! The covering has its own glue on it. If you CA it you will have a hard time getting it off if you need to patch it again!
If you just try to stick the patch on over the old covering, it will sag when you put the iron on it and you probably won't get it stuck down right.

BTW you will want to clean off all the oil before you try to stick any more covering down.

Fixing a patch in a wing
 

The old covering. If the edges of the new piece of covering line up with ribs & spars the seam won't be as visible.
If the hole is small (less than about 2 in each way), you might be able to just put a new piece overtop. If hole is bigger it will most likely be too hard to get a new piece to look nice.

Fixing a patch in a wing
 

The wood itself doesn't have to be exposed, covering will stick to covering (although overlapping large areas can create bubbles). The idea is that there should be something solid under the patch. Trim it to within 1/4" - 1/2" then fold down the extra.