If the hole is about the size of your fist, you can get sheets of this self adhesive webbing that you cut to fit. It is just this plastic material. If the hole is big, these sheets come in squres with a metal material that adds support. Just sand around the hole to get a good flat surface , apply the webbing then apply 1 coat of plaster to cover the patch and surrounding area. You will probably still have a sunken area where the hole was. Once this layer dries, you can sand down the area, doesn't have to be perfect, then apply the second coat of plaster to fill in the sunken area and make all other surfaces even. Once this dries, sand all areas to the same, smooth level.