Clean the wall with a clean sponge and warm water. This will remove any glue residue that was left on the wall. Avoid rubbing too hard, and use long strokes. Allow the wall to dry completely. Test this by touching the wall. If it feels cool to the touch, there is still moisture in the wall. Leaving the wall to dry overnight is a good idea.
Apply joint compound with the joint knife. Smooth on all seams in the sheet rock, as well as in any gouges or nicks in the sheet rock's surface. Allow to dry according to the instructions on the bucket of joint compound. Some dry within a few hours, while others require overnight drying.
Use the joint compound sanding mesh to even out the patches of joint compound. Sand gently in either an up-and-down or left-to-right motion. Do not sand in circular motions. Sand gently until the patches of joint compound are smooth, flat and flush with the rest of the wall.
Rinse the wall once again with the sponge and water. This removes any joint compound powder that's on the wall. If you don't wipe down the wall at this point, any primer, paint or wallpaper you put on the wall will not adhere properly. Allow the wall to dry overnight.
Apply two coats of primer to the wall. Follow the directions on the can of primer for drying time. Some dry in as little as an hour, while others require 8 hours of drying time.
Finish by adding the wallpaper or paint of your choice.