Take off all hardware from the door. Door knobs and knockers usually pop off the door easily if you remove the screws holding them in place.
Remove any peeling paint from the door with a paint scraper.
Dip a putty knife into a can of wood filler and apply it to any holes or depressed areas in the surface of the door. Even out the surface and let the wood filler dry for the amount of time specified on the product label.
Sand the entire door with 120-grit sandpaper to get smooth, even surfaces. High-gloss paint will make imperfections seem more obvious, so the preparation steps are important.
Vacuum the door to remove sanding dust. Wipe all surfaces with a tack cloth to get rid of any remaining dust.
Apply primer to the door, starting with any molding around the door panels. Continue with the center of the door panels, working outward until you cover all the panels. Paint the rest of the door, starting from the top. Let the primer dry. This coat helps the paint adhere to the door.
Sand the entire door surface gently with 220-grit sandpaper to smooth out the surfaces.
Vacuum and wipe the surfaces again to remove sanding dust.
Brush on a coat of the high-gloss paint, following the same sequence as with the primer, and let it dry. Apply as many coats as it takes to get the color saturation you like. You may have to apply three or four coats of paint.