Well, without being a pro at it:
It somewhat depends on how the surface looks today?! If it's a "torn" surface, I've personally had trouble making it look nice. If you knock off the loose parts and fill in with new plaster and scrape off with a trowel or sponge (the latter to get a smoother surface), it's easy to scratch the surrounding, old area. I would probably apply a thin, thin layer over the entire surface if the overall appearance was important.
Then there's the usual, water thoroughly and use the same or weaker plaster for the repair.