It's not the easiest job to start on but basically get a patch of lining paper, soak it for five minutes with paste, place it over the damaged area and cut with a very sharp clean blade through both layers of paper, then take the old paper out from underneath the new piece and paste the new piece in.
Unless you are some kind of wallpaper savant this is not going to work first time out but you'll get close. Fill the gaps between old and new paper with a decorators filler.
Toupret ready mixed (red pot) is pretty useful, or TX110 from Toupret if you don't mind mixing it. Of course you'll have to buy way more than you need. When you fill it, let it dry off a bit and use a sponge to clean up the excess instead of sand paper.
It's not the easiest job to start on but basically get a patch of lining paper, soak it for five minutes with paste, place it over the damaged area and cut with a very sharp clean blade through both layers of paper, then take the old paper out from underneath the new piece and paste the new piece in.
Unless you are some kind of wallpaper savant this is not going to work first time out but you'll get close. Fill the gaps between old and new paper with a decorators filler.