Priming
In most cases, wallpaper adheres more easily if primer-sealer is applied to the wall. This compound keeps the wall surface from absorbing moisture from the adhesive and also protects the wall from damage if the covering is later removed. Walls painted with a high-quality flat alkyd paint may not need a primer.

Choose a primer-sealer designed as a wallcovering undercoat. Apply it with a roller at least 24 hours before hanging the wallcovering to allow it to dry thoroughly. When you're hanging a light-colored paper over existing wallpaper or a colored wall, use pigmented primer-sealer matched to the paper's background color.

Sizing
A liquid coating designed to make wallpaper apply and adhere better and go on more easily, sizing is thought unnecessary in most cases due to advanced technology in primer-sealers. You may want to consider sizing if you're hanging a porous or heavy paper, if the wall is textured or alkyd painted, if the paper isn't sticking well, or if the wallcovering manufacturer specifically recommends it.

Apply sizing with a roller or brush. It dries quickly, so you can begin hanging the wallcovering immediately unless the manufacturer recommends otherwise.