Hi,
had to do one myself not long back, not the most pleasant of jobs.
Firstly, lightly sand the surface to provide a key (wear goggles and a mask).
Optional, but I always do, clean the surface with some sugar soap and mutton cloth.
One coat of good quality (oil based) undercoat - the sort usually used for wood. It's essential that you have plenty of ventilation as oil based paints stink when used over a large area.
Then finish with a coat of emulsion (silk or matt).
Hope this helps.