Take the door off the hinges by using a flat head screw driver to wedge between the bottom hinge cap and lower hinge. Tap the screw driver with a hammer in a downward motion until the cap becomes loose enough to remove. Place the Phillips screw driver into the space where the cap was, and tap the hinge pin in an upward motion so it becomes loose. Remove it from the top of the interlocking hinge teeth. Do this for all the hinges and remove the door.
Remove the doorknob striker plate and doorknob with a screwdriver and set the hardware aside.
Sand new doors that have never been painted. Use a 120 or 150 grit sand paper. If you use an electric sander, use light pressure. Your main goal is to remove any bumps and imperfections. Do not over-sand a spot if you cannot remove or sand down an imperfection, as this could damage the wood.
Clean previously painted doors with soap and water. If the painted door is heavily soiled, use products with trisodium phosphate to clean it.
Apply liquid sander to cleaned doors. Remove any remaning paint chips with a paint scraper. Fill any dents, gouges and cracks with spackling paste and smooth over so it's even with the door. Sand the repaired area until it's completely smooth.
Paint the door with primer with a paint roller. Primer helps paint to adhere to the surface. Use a smaller brush to paint the sides of the door. Allow to dry completely.
Apply paint to the door using a roller. For best results, apply two thin coats of your chosen color, rather than applying one thick coat. Use a smaller brush to paint the sides of the doors. Let dry completely between coats.
Touch up any spots you missed with a small brush. Let dry.
Re-attach the doorknob and doorknob striker plate and mount the door back onto the hinges.