Take the door off its hinges if you can. Painting a door flat on trestles gives you an even finish with no runs, and it lets you reach the edges properly. Leaving it hanging saves you a day and costs you the top and bottom edges.
Strip the furniture off rather than masking it. Letterplate, knocker, handle, numerals. Masking tape around a letterplate leaves a ridge of paint you will look at every day.
Sand back to a sound surface. Flaking or blistered paint has to come off, not get painted over. Fill any splits, sand flush, then dust the whole door down.
Prime bare wood. Any timber you have sanded through to needs primer before it takes topcoat. Knots want a knotting solution first, or the resin bleeds through the finish within a year.
Two topcoats, thin rather than thick. Use an exterior paint made for timber. Microporous systems let the wood release moisture instead of trapping it under a film, which is what causes blistering.
Paint the top and bottom edges. This is where most repaints fail. Both edges are end grain, both are out of sight, and both drink water. A door that swells and sticks in winter is usually a door with an unpainted bottom edge.
Leave the door off overnight if the forecast allows. Rehanging a door before the paint has hardened prints the weatherseal into the finish.