Unscrew the door from the door hinges.
Clean the door by wiping it down with trisodium phosphate or mineral spirits and allow it to dry.
Place painter's tape around the edge of the door handles.
Remove the paint from the door with a chemical stripper, if it has several layers of paint on it. Apply the stripper to the surface of the door, and allow it to react with the paint for the time specified by the manufacturer. Scrape the paint off of the door by sliding a putty knife along the surface. Be careful when using the putty knife so you don't damage the wood. Use a toothbrush to remove the paint from places inaccessible with the putty knife.
Fill any holes in the door's surface with wood putty. Sand the wood putty and any rough edges with fine-grit sandpaper until the surface is smooth. Wipe the dust off of the wood with a clean, dry rag.
Coat the wood with primer. Prime small, inset areas on the door first, using a 1-inch brush, before moving onto the larger sections of the door, using a 2- or 3-inch brush. When applying the primer on the larger areas, go in the direction of the wood you're priming. Allow the primer to dry.
Paint the door with the new paint color, using the same technique you used when applying the primer. Allow the paint to dry, and add a second coat if needed.