Clean any holes or cracks and prepare them for patching. To do so, first clear away any loose or crumbling pieces of plaster using your hands, then sweep away dirt and debris using a wire brush.
Blend a plaster repair mixture with water according to the instructions on the package.
Cut a piece of fiberglass mesh tape to fit each hole. Place the tape in the hole and immediately cover it with the plaster repair mix, applied with a putty knife. If the hole is deep, apply the patching compound in 1/4-inch layers, allowing each layer to dry before applying the next.
Wait 4 to 6 hours for the patched areas to dry, then gently sand them using a medium grit sandpaper until they are level with the rest of the wall surface.
Prime your walls using an acrylic primer. Apply the primer using a regular paint roller for the best results. This gives the wallpaper adhesive a solid base to adhere to.