Painting doors can be a little difficult, especially if you choose to paint them while they are installed. For the best result the door should be removed, stripped of all hardware, and placed in a horizontal position on two sawhorses before painting. This will provide a much better working position for applying the paint and a much better outcome.

How to paint a six panel door:

Six panel doors are quite common as entrance and interior doors. Often the front door is painted an accent color to draw attention to the entrance of the home. The six panel door design has six raised sections to paint. If the door is laying flat it is much easier to paint each of these sections without having the paint running.

Painting tips for panel doors:
Remove the door from the door frame and then remove all the door hardware. Painting a door while it is installed in a vertical position is almost certainly not going to come out looking like a professional paint job.
Check the door casing to insure that it is in good condition. If there is old dry paint on the casing it should be sanded before painting.
Check the door for dry paint and sand as necessary before painting. If the door is made of metal, use a primer paint on any shinny areas of the metal before applying the finish paint.
Paint the recess panels first and then the horizontal and vertical rails that separate the panels.
Prevent lap marks caused by painting over partially dried paint by painting the two top panels first, then the top rail and continue down the vertical rails. Then paint the middle panels and middle rails. Finish up by painting last the lower panels and lower rails. Paining in this order gives you a better chance at achieving a uniform finish.
Let the side dry and turn the door over on the saw horses and paint the opposite side.
Finally, paint all the door edges.

That is how to paint a six panel door and have a professional looking finished paint job.