the way i do is to cut a piece of plasterboard the same width as the hole, cut it to length but add half inch either side. poke a long nail through the plasterboard and put some board adhesive on both sides of the board. put the board in the hole and pull the nail and it will stick to the back of the plasterboard. fill in the rest of the hole with board adhesive so its nearlly flush, let it set then patch in with plaster, hope this makes ant sort of sense to you.