First, smooth the damaged wallpaper as much as possible.
Find a piece of wallpaper with a matching pattern and tear a patch so that the underlying paper is not showing.
Moisten the back of the wallpaper to activate the adhesive. If the wallpaper is not prepasted, apply a thin layer of wallpaper paste.
Position the patch so that it matches the surrounding wallpaper pattern. Press the patch into place with a sponge.