Lightly sand the old paint of the door with a mild grit sandpaper to prep the door for painting.
Tape off the door knobs and hardware with painter's tape to prevent accidentally painting areas that you do not want paint on.
Place a drop cloth underneath the door to catch all drips of paint that may fall. Make sure that a door stop is in place behind the door to prevent the freshly painted door from touching the walls.
Decide which side of the door to paint first and use a second door stop or block on the front side of the door to hold it in place while you apply paint. The point of the door stops is to completely immobilize the door while you paint it.
Pour paint into the paint tray and use a paint roller to apply it to one side of the door. Completely cover the door in paint. Use a paintbrush to get tight spaces where the roller will not fit properly. Allow the paint to dry.
Move the door stop to the other side of the door and paint that side. Apply a second coat if necessary after first coat dries. Remove the painter's tape and drop cloth once door has dried.