Always take precautions when working with primer and paint. Open up any nearby doors and windows to let in fresh air. Consider wearing a respirator so you don't end up breathing in fumes. Use painter's tape to cover up these parts. Stick the tape directly to the door knobs, around the area that attaches to the door. Tape over the hinges to protect them. Also apply tape around the outside edge of any glass panels in the doors. Taping the windows is tedious. If you are using latex paint and don't mind scraping paint off them later, you can leave the windows uncovered. The lock set is the metal piece on the frame's side next to the door knobs. You will need a Phillips head screwdriver to remove it. Loosen it by turning the screws counterclockwise. Slide the lock set out, then wrap tape around it. Push it back into the door and screw it in place. Pick up a bucket of paint primer from a home improvement store. Dip a brush in it and spread it onto the doors. Move slowly to paint on an even layer. Read the label to make sure the primer is compatible with the paint you intend on using. You cannot use a latex primer under an oil-based paint. Check the manufacturer's instructions on the label for the recommended drying time. Primer takes about 3 hours to dry completely. You can leave it longer to ensure it dries, but paint as soon as possible to limit the amount of dust settling on the doors. If dust settles on the door, wipe it off with a dry cloth before painting.
Ventilate your area and wear a respirator. Tape around the door knobs, hinges, and glass. Unscrew the lock set to tape around it. Spread the primer on the doors in short, even strokes. Let the primer dry completely before painting.