Remove the door handles from the door. Remove the doors from the hinges and lay them flat on a tarp.
Clean the doors thoroughly with an ammonia-based cleaner. Apply the cleaner to the door and wipe it away with a clean rag. Make sure all surface grime is removed from the doors. Oils and grease left behind will interfere with the paint and cause it to peel and bubble.
Sand the doors with medium-grit sandpaper. Apply firm pressure to the surface and sand in the direction of the wood grain. Sand the front side of the door, then turn the door over and sand the other side. Wipe away all sanding dust with a damp rag.
Apply primer to the door with a good quality paintbrush. Primer will prevent the original color of the door from bleeding through the paint. Primer also provides a better finished product by creating a smooth, even surface as a foundation for the paint. Apply two coats of primer, allowing the first coat to dry to the touch before applying the next. Allow the final coat to dry for four to six hours. Flip the door over and repeat the process on the other side.
Paint the doors with interior latex paint. A semi-gloss paint will allow the doors to be wiped free of fingerprints and other dirt while still allowing for some mistakes while painting. Higher gloss paints will show every brush stroke. Apply two to three thin coats of paint and allow the final coat to dry overnight. Turn the door over onto the opposite side and repeat the painting.
Hang the door in its original location and replace the door knobs.