When you’re ready to paint, remove all hardware and clean the door REALLY well. Then start by sanding your door down as much as possible. It’s not necessary to strip all the existing paint off, you’re just trying to roughen up the flat surfaces of paint. Hand sand the areas with molding or crevices and then wipe the door clean. Elevate your door by using two 5 gallon buckets works great, and allows the paint to dry more smoothly than painting your door while its vertical.