Summarize this article in one sentence.
Using a roll of painter’s tape, slowly work around the sides of the door and cover each edge. This will help keep the paint only on the face of the door and make the lines around each edge as clean as possible. Trimming the door with tape might be difficult around long or uneven edges, but it will make the painting process much easier in the long run. If there are any parts of the door you can’t remove, such as windows, you can cover these with painter’s tape to keep them clean. Before painting the door, take the opportunity to fix any dents on the door’s surface. Use 80-grit sandpaper on any dented areas before smoothly covering the dent with a small amount of patching compound or auto body filler. Leave it to dry for around 40 minutes and then sand it with 150-grit sandpaper to make the area level with the rest of the door. Sand the door until the dent is no longer obvious or not easily visible. Painting over it will help obscure any minor dents or dings, so don't worry too much about using a bubble level To allow the primer and paint to adhere to the surface of the door, you should sand it down lightly first. Use fine grit sandpaper around 400 over the entire surface of the door. You don’t need to sand the whole surface of the door thoroughly, only enough to give the primer something to stick to. Applying too much pressure or using a rougher sandpaper can risk damaging the door too much. Sanding can create a lot of dust that can get caught in the paint and affect the look of your finished door. Slightly dampen a clean cloth and wipe down the surface of the door to remove any dust left over from sanding it down. If there is a lot of dust, or if the old paint has come loose in the sanding process, use a vacuum cleaner to remove the majority of the dust before wiping it down.

Summary:
Tape off any areas that you don’t want to paint. Fix any dents in the door's surface. Sand the entire door with 400-grit sandpaper. Wipe down the surface with a damp cloth.