The first step in the repairing of an existing drywall is to patch up the holes with a joint compound. This highly relies on the number and the dimensions of the holes that have appeared with the passage of time. A simple trowel with joint compound can fix small holes but if the holes are big and require greater treatment, then you will have to stuff the hole with a piece of paper first and then apply the compound as only then it will be able to resist against it.
Now pack the holes tightly and cover them with a mesh of old wires.
The next step involves the sanding of the rough edges that have appeared. Using a medium grit sand paper is the best possible choice for the above matter but to do it in an even better way would involve the usage of a palm sander.
Now you will have to cover all the wall paper with the joint compound and go over every bit of the surface. To avoid the seams from showing, sand the seam before and after this step.
Next you will have to make it look presentable by painting the walls with the proper paint combination for going over the existing wallpaper. Do it twice for achieving even better results. After this step, the painting has been done and the wallboard will now look as new as ever. Just let the painting works dry with time so that you achieve the best possible results. You can also use a fan for achieving the drying quickly but it is only advised of you are in a real hurry to fix your drywall.