You have to shim and fuss with it, it doesn't get any easier. To fill large voids we use what we refer to as a hot mix for large fills, mix plaster directly into your joint compound and it will dry very quickly. Or use straight plaster so you can work with it more quickly. By varying your hot mix you can do a repair within one afternoon, always finish up with a skim coat of straight joint compond so you can sand it. Also use tape where is appropiate, Home Depot sells a new paper tape with holes in it that we like. You may need to tear out more plaster if it is falling away, on some occassions you can resecure it with some screws.
This is a fairly advanced repair, if you are a beginner you may find it difficult.