I'm in the process of repairing the same type of damage. I had quite a few brown paper patches in my kitchen after I removed wallpaper that had been placed directly on untreated drywall. I used joint compound diluted with water to the consistency of pre-made cake frosting. Then I used a 6" trowel spreader and skimmed the damaged area in a thin layer of the compound. It was much easier than I had imagined, even fun. Some areas required a little more work than others but I was able to do my entire kitchen in a couple of hours. The next day I sanded it and touched up a few places. The last step before painting was the primer coat. Believe me, if you've removed the wallpaper you've done the hardest part already.