Purchase some joint compound and spread it over the affected areas. After it drys sand with a fine sandpaper on a board until it blends into the bad areas. This may require a second coat. The sanding is the most important part since it will dictate how smooth the wall looks. Paint will show the slightest imperfection.

Hi kuiperp & hi Dr home

Just some additional suggestions.Before you apply your initial "coats" of joint compound..make sure the "rock" is sound not crumbling out.And also like Dr Home said,paint reveals just about every imperfection on your wall surface.What I like to do is after you've finish sanded the patched areas and they look good...hold and incandescent light to every side and look for ridges and high spots...once you paint they'll stick out like a sore thumb.Also once your satisfied w/your patches I'd consider "skim" coating your walls w/thinned joint compound applied w/ a heavy knap roller then trowel it off-it smoothes out your wall and fills overlooked imperfections..as well as giving your walls that "plastered look" when they're painted..have fun!