As with most jobs - There's a right and wrong way IMO. For a door thats been previously varnished, rub it down with some medium grade Aluminium oxide paper, wrapped around a cork rubbing block to "break" the surface and give a key for the paint to adhere to(take it you don't have access to a power sander) clean off dust with a "Tac" cloth or cotton rag (ideally something lint free ie not paper towel dampened wth white spirit, then undercoat and topcoat. Depending how fussy you are about finish, I would recommend a light sanding with fine paper between coats.
And this thread cropped up today coincidentally on another forum I look at. I find oil based paints easier to work with - good luck
http://www.diynot.com...iewtopic.php?t=261140