The first stage is to give yourself something to fix a new piece of plasterboard too. I have just used some old pieces of wood, which I've screwed into place either side of the hole with plasterboard screws, that have been sunk slightly into the surface.
Next you'll need to draw a template onto your plasterboard that's the same size and shape as the hole. In this case my hole happens to be the same shape andalmost the same size as a 1L tin of paint (its actually about 1mm smaller than the tin).
Cut the shape out using a plasterboard saw. If the shape is square you can use a straight edge and trimming knife.
Insert the plasterboard into the hole and attach with plasterboard screws to your pieces of wood. Cover the joins between old and new plasterboard with scrim tape (flexible fibre glass tape), to avoid any future cracking
Apply 2 - 3 coats of filler (I use easi-fill). Make sure you allow each coat to dry fully before applying the next. Its best to apply several thin layers, rather than one really think one. It'll dry quicker and sand out nicer.
The final stage is to sand the filler and apply your emulsion.