Before you begin, remove everything you can from the room. Any furniture left in the room should be moved to the center of the room and covered with drop cloths. Cover flooring with drop cloths for protection from spills. Remove any fixtures and hardware on the walls.
Remove any existing wallpaper from your walls using a wallpaper scraper tool and Dif liquid wallpaper stripper.
Fill any cracks or holes in the wall using a joint compound and drywall compound or spackle. Sand any patched areas with a palm or circular sander until they are smooth and coat them with interior primer. Let the primer dry completely. If the walls are painted with a semi-gloss or gloss paint, give them a light sanding to dull the surface so the glue adheres better. Be sure to sand away any tiny bumps on the wall before hanging wallpaper — bumps will show through wallpaper. Run your hand over the sanded surface to make sure it is smooth.
Apply a coat of wallpaper sizing to the wall if necessary. This will help you slide the paper into place when hanging it on a wall with flat finish paint. Apply the sizing to your walls using a paint roller. It's inexpensive and makes a better surface to hang wallpaper. If your walls have been primed, you can skip this step.