The easiest way to paint a door is to remove it from the door frame. Using a screwdriver, take off all of the hardware on the door including hinges, door locks and handles. Place the door on sawhorses and then you're ready to paint. If it's an old door, you may want to give it a light sanding before you start.
Make sure you stir the paint tin well before use. Start by giving the edges of the door an undercoat with a brush. Then fill up a tray with the undercoat and draw a roller along the tray's grate two or three times to apply the paint evenly to the roller. Apply the paint to the door in a ‘W' formation. This will make sure that you'll have an even covering of paint.
Apply the topcoat using the same process as the undercoat. Use a brush around the edges of the door and roll the main surface of the door with the roller using the ‘W' technique. Don't forget to ‘lay off' the topcoat for a great finish.
When that side is dry, flip the door over and repeat steps 1–4 on the other side. Once it's dry, reattach the door fittings and hang the door back on the door frame and you're done.