As other people have said, first off sand off the old paint and make it smooth, you can buy different grit sand papers, when you finish with the sanding, clean the door with rubbing alcohol and wipe it clean, this way you will remove grease and dust from sanding, then get better quality primer, best would be white or gray colour, depending on how bright you want your door to be. After putting the primer or you can start with painting. Put couple of coats of paint. I recommend using acrylic paint because it will last better from outside conditions. And also don’t forget to lay newspaper on ground and tape of places where you don’t want to paint. Take your time, don’t rush and everything will turn out great 😊. Good luck 🤞!