Wipe down the door with a de-greasing soap. This should clean the door so it is free from dirt, debris and grease.
Remove the door handle and lockset from the door with a screwdriver. Set aside the hardware and screws for later reinstallation.
Apply painters tape to the hinges. This will protect the hinges from getting coated with paint as you work.
Prime the steel door with latex painting primer. Use a short nap paint roller and paintbrush to coat the door with the primer. Wait for the primer to dry -- usually one to two hours -- before painting.
Roll the latex paint onto the door with a short nap paint roller. Use a paintbrush to reach into the details and molding of the door. Let the paint dry and then re-coat.
Allow the paint to dry. This will take two to three hours.
Re-install the door hardware with a screwdriver.