This tutorial assumes that you are painting the door side that opens into the room.
I recommend that you paint the door in place ( do not remove from hinges).
To prep the door for painting, lightly sand the surface with 120 grit sandpaper, a sanding sponge ( available at most hardware or paint stores ) works best for the curved areas inside panels. The key word here is "lightly" do not sand through the existing finish or round the edges.
After sanding clean the entire door with a mild detergent mixture. a few drops of dish washing liquid in a pail of water works fine. Pay particular attention to the area around the door knob.
After cleaning is complete wipe down entire door with a clean damp cloth (with clear water). Let door completely dry.
If you are using a Latex paint we recommend you use a latex paint additive called a "wet edge extender" to
keep the paint from setting up too quidkly while applying the paint.
Loosen the door knob, usually two screws located on the face plate. This will allow easier access while painting around the door knob and it will still be useable. Start painting with the door closed. Close the door, if you can see the hinge pins you are at the starting side.