is easy if you follow a few steps. Is it a new door? Wash it with a degreaser, get all of those black fingerprints off it! Lightly scuff it with scotchbrite or similar product.
Purchase a Good 2 1/2" sash brush, no not a $5.00 cheapie, spend some money and buy a good brush $20&up. Also buy a trim roller, about 1"around and 4"long. Purchase a good acrylic primer, a quality acrylic top coat and a qt. of flowtral paint conditioner. Mix the flowtral into the paint at the recomended rate. Lets see if I can get you lost? Use the roller to paint the panels from left to right top to bottom. Brush them out. Roll the short vertical stiles between the pannels, and brush them. From the top of the door working to the bottom roll out the rails, and brush them. Now roll out the stiles on outside of the door and brush them. Your done!
The edges should be the interior color on an inswing door and the exterior color on an outswing door.