How to fill a crack in my wall?

Fill in cracks and holes with plaster or another appropriate filler. Use a table knife to help you work the filler into the crack then use the flat side of the knife blade to smooth the surface of the repair. Once the repair is dry, use sandpaper to lightly smooth the surface. Remove any dust from the wall with a rag. Remaining dust can keep the wallpaper from adhering to the wall.