The difficult things about painting a door when it’s vertical are the drips and runs that always happen. If you take the door down and paint it horizontally, you solve that problem. But that means you have to wait for one side of the door to dry before you can paint the other. The solution is a simple one.
Drive two three-inch screws into the bottom of the door near each edge, leaving about two inches exposed. Then drive one three-inch screw into the top of the door at the center leaving the same amount exposed. Rest the exposed part of these screws on a sawhorse at either end of the door.
When you finish painting one side, you can simply lift the door by the two screws on the bottom and pivot it on the single screw at the top to continue painting the other side.