If your ceiling has been painted before, you won't need an undercoat. Brush it down with a soft broom to remove any cobwebs. Remove any flaky paint, fill any holes and give it a light sand.
If the ceiling is new and has never been painted, just make sure it is clean. There's no need to use a special undercoat, you can go straight ahead with the ceiling paint. It has a flat finish that hides small imperfections.
If you're not planning to paint the walls later, mask off around the tops of the walls with masking tape.
Start by laying out some drop sheets in the area you need to work. When you open your tin of ceiling paint, you might notice that the paint has separated. Use the paint stirrer to stir the paint thoroughly, so that all of the pigments are blended back to together.
Pour some paint into the roller tray until it reaches the bottom of the grate. Attach the roller to the extension pole and dip the roller into the paint. Pull it up the grate two or three times until the roller is evenly coated with paint. Makes sure not to put too much paint on the roller or it will drip when you use it.