Removing the backing paper and then applying the whole sheet to a cover
is very tricky.
The best and quickest method I have found is to sandpaper the backing
paper. Sandpaper an inch or so in from the edges and then some spots in
the middle, no need to sandpaper 100% of the surface. Then apply
archival type glue. Since the backing now has a rough surface, it will
stick to the envelope easily and not peel.

Cheers,

Paul