Remove the door from its hinges. Push the hinge pins out by inserting a nail into the bottom of the hinge channel. Tap the nail lightly with a hammer so the nail pushes the pin out of the hinge. Lean the door against a wall after removal.
Wipe down both sides of the door with an ammonia-based cleaner to remove surface dirt and grease. Wipe completely dry.
Sand the door lightly with 120-grit sandpaper or a sanding sponge. Remove the surface sheen from the old paint so the entire door looks dull but feels smooth.
Smooth inside corners and contours on the door using the corner of a sanding sponge. Pay particular attention to areas around panel insets. Wipe down the door with a slightly damp rag to remove all the dust from sanding.
Paint the door with an oil-based primer. Use a narrow brush to paint inside contours and corners first, applying a smooth, even coat. Use a wider brush or a small paint roller to paint the primer onto flat areas of the door. Allow one side of the door to dry before flipping it and painting the other side and edges.
Sand the primer coat lightly with 120-grit sandpaper after it dries completely. Sanding removes slight unevenness in the primer and roughens the surface slightly so the final coats of paint adhere better. Wipe the sanding dust from the door and paint it the desired color.