Wallpaper hangs best when walls are clean of debris and the surface is smooth. It is important to take the time to remove old wallpaper, fill in any cracks, and repair imperfections in the walls surface. This will help the wallpaper to lay flat and avoid potential problems with the paper adhering to the wall. If it is a painted wall, you will need to smooth out the walls with sandpaper. Once all repairs have been made and your walls are smooth, you will want to clean them up. Use a sponge and a little soap to remove dirt and grease. Rinse the wall with clean water and allow the wall to dry before proceeding. When walls are clean and smooth you are ready for the next step which depending on your project may include sizing, priming, or installing lining paper.