Dings, dents, holes, and gouges in your interior walls can be fixed with moderate do-it-yourself skills, but first you have to know what kind of walls you have. Up until the 1940s, interior walls were finished with plaster. A plaster wall consists of wooden slats or metal mesh that supports a layer of Portland cement. This is troweled by hand onto the walls, making it an expensive and labor-intensive process. More modern homes have drywall: large, flat sheets of pressed gypsum that are nailed or screwed in place, with the seams covered in paper tape and joint compound. Small holes or cracks in either type of wall can simply be filled with spackling compound, using two coats if necessary. Larger holes require different techniques for the two types of walls. For drywall, draw a rectangle around the hole and cut it out. Next, cut a piece of wallboard four inches wider and four inches longer than the hole you just made. On the back side of the patch, draw a smaller rectangle two inches in from each side. Now carefully cut through the back of the patch along the lines, taking care not to cut all the way through the face paper. This paper flap is what will hold the patch in place. Finally, put joint compound around the edges and sides of the hole and slide the patch into place. Add extra joint compound if necessary to coat the underside of the flap completely. Smooth it with a putty knife and let it dry. Then apply another layer of joint compound if necessary. To patch plaster, start by removing all loose plaster and beveling the underside of the hole so it's larger than the exterior. Fit a piece of metal lath into the hole and nail or staple it to the wood laths. Dampen the area, and then apply a first coat of plaster to fill the hole. Make scores in this layer that will help hold the next layer. Let it dry thoroughly. Moisten the area again and put on the second layer of plaster, feathering it to match the surrounding walls. When this dries, you can finish the area with joint compound.