If the wallpaper has separated from the wall, all you need is a field knife, wallpaper glue and a brush. With the help of a knife, you need to lift the wallpaper parts where the glue has released, and then lubricate it with fresh glue under it with the help of a brush. Carefully smooth the smeared part back into place, then excess glue poking out on the edge of the wallpaper can be easily removed with the help of a damp rag.
If, after drying, bubbles remain under the wallpaper, a transverse cut should be made with the knife. The resulting tongues should be folded up enough to be lubricated with glue. Once all four tongues are smeared, fold them back into place and then go through them a couple of times with the help of a roller. Excess glue must be removed in the already mentioned way.
If there are crumpled tears on the edges of the wallpaper, you need to lift it, coat the back side with glue, wait about 1 minute for the paper to soften. Once you feel that the edge of the wallpaper strip is not hard, you can gently smooth it back to the edge, taking care not to tear the paper. Using an edge roller, the torn strip of paper can be perfectly reinserted. If we have done these tasks skillfully and with a steady hand, it will be almost impossible to notice previous errors and repairs.
If a larger surface needs to be repaired, it is worth finding a larger piece of old wallpaper. Cut a piece large enough to perfectly cover the defective part, then temporarily glue it to the wall with adhesive tape so that the pattern fits exactly. (In the case of one-color wallpaper, of course, it is easier.) Take out a ruler, and then, with the help of a sharp knife, cut a square or rectangle through the old and new wallpaper. Using a spatula or scraping knife, remove damaged wallpaper from the wall, preferably without wetting this process.
After the surface is suitable for sticking new wallpaper, lubricate the back of the stain with glue and immediately fit it into place. Use the edge roller to smooth over the new wallpaper a few times. There is no need to be alarmed if small bubbles remain on the surface, as they will disappear and smooth out after drying.