Locate an inconspicuous area you can test. Perforate the wallpaper with a wallpaper perforator. A perforator looks like a pizza cutter, only the wheel has small pikes. The spikes cut holes in the wallpaper, exposing the wall behind the paper. Perforate a 1-foot square area.
Apply a 1/4-inch layer of plaster to the surface of the drywall you perforated. Use a trowel. Allow at least 24 hours drying time. If the wallpaper peels off, you must remove the wallpaper before you texture the wall. To do so, spray the wallpaper with a spray bottle filled with hot water, then scrape off the paper with a wallpaper scraper. If the wallpaper did not peel off the wall with the plaster on it, you can forego removing the wallpaper.
Perforate the entire. Make sure you adequately cover the wall. The more perforations you add, the better the plaster will stick.
Apply a ¼-inch layer of plaster onto the wall with a trowel. Allow 30 minutes for the plaster to set up.
Roll over the plaster with a clean lamb's wool paint roller to add texture to the surface of the plaster. Replace the roller with a new one as the plaster builds up on the roller.
Allow at least 24 hours drying time before you paint the wall surface.