A simple enough fix really. Mark out the damaged area, and square it up. Once you have marked it out, cut the whole area of plasterboard out with a pad saw. Once you have removed the damaged area of p/board, you'll need to put in and fix some timber noggins to screw the new piece/patch of plasterboard to. Measure and cut the new piece of board, then fix it into the ceiling, screwing it to the extra noggins you put in earlierMake sure you use the correct thickness of plasterboard, either 9.5mm or 12,5mm. Once you have fitted the new board, you can either tape the joints around the patch and blend/feather it into the the original ceiling, or tape the joints and skim the whole ceiling. The best way is to skim the whokple thing and you'll never see the patch.
