You will find that using a cold chisel is the easiest, quickest and produces the least mess.
For the first timer, use a long level of something similar and a Stanley knife and secure the edges of the groove you are looking to make. This helps to stop the plaster cracking when your knocking the rest out.
Down the centre of the groove, and not near either side you have scored, run the cold chisel (electricians bolster is a great tool c£5) down with a steady tap, your looking to create a small recess to begin within, not take anything out. Now, just inside your two score lines run the cold chisel down each side, a light tap as you go with a hammer. The plaster should now start to crack to the centre. Once you have the skin off, it's very easy and you can be a more forceful with your taps. It's just like making a recess in wood with a chisel.
Once you have the hang of it, you will work out which bits you can delete.
The golden rule to begin with is tap gently. If the initial taps don't start to remove the top skin of plaster, go back and tap harder down the centre and then the sides. Don't just hit the sides harder, you need to make somewhere for the plaster to go, otherwise you will 'blow' the sides of the grove.