The best way to do it would be to get a plasterer to do it, but you could save a few quid by having a go yourself.
Firstly block the holes, preferably with plasterboard which you could hold in place with another bit of plasterboard over the top, or glue. Tape over the joints with scrim tape and cover over it with finishing plaster or polyfilla trying to smooth it off as best as you can using a plasterer's trowel.
Leave it to dry for a couple of days, then paint over it.
Adam
December 2009