Start with a slight rub down and then apply a good quality primer undercoat. Start by painting the outer edge of the door.
Then continue by doing the top two panels, then the centre panels and the last two panels, working your way down.
Then start with the top section of the door above the panels, then the centre vertical piece, then the horizontal section, then the vertical section and so on, and then finish off by painting the vertical sides top to bottom. Then you check the corners of each panel for any runs. This way you should avoid any runs, if applied evenly with a good quality paint brush.