To apply new wallpaper over existing wallpaper, cut off peeling pieces of the old paper, apply adhesive to the back of the new paper and lay the new paper on the wall. Use a smoother to work out bubbles.
Remove all nails from the wall, and wash the old paper with a weak solution of dish detergent and water. Allow the paper to dry completely before cutting off pieces of bubbled or peeling paper with a utility knife. Use nail hole filler to create a smooth surface, sanding the repaired areas. Using a level and a pencil, draw a vertical line down the middle of the wall away from existing wallpaper seams. Cut the first strip of wallpaper with 2 inches of excess on each end, and apply a coat of adhesive across the back, before rolling the strip adhesive-side out.
Starting at the top of the wall, unroll the paper while pressing the adhesive to the old paper. Use the smoother with even, gentle strokes from top to bottom to work out bubbles. Repeat the process for subsequent wallpaper strips until the whole wall is covered, ensuring the edges are flush but not overlapping. Use a utility knife to trim off the excess paper, and run a seam roller along the edges for a tight fit.