Remove the door before painting. Use a slot screwdriver to remove the hinge pins and then remove the other half of the hinge from the door, so that paint won’t get on the hinges. Remember the front door will be heavy once the hinges are no longer holding it in place, so get help to lift it onto some sawhorses.
Take any other hardware off the door, including the door knocker and the peephole, which is usually two snap-together pieces. Also make sure to remove the doorknob. Usually, this involves taking out four screws, two that hold in the latch and two that hold the doorknob itself.
Prime all six sides of the door, including the inside, the outside, the left and right edges and the top and bottom. Any surfaces not primed will absorb moisture.
Visually inspect the door for surface cracks and repair with a caulk gun and a putty knife. Let the caulk dry and re-prime the spot.
Use a fine-grit sandpaper, such as 180 grit, to sand the surface smooth of paint drips and the like, particularly in the crevices of the panels. Then use an old paintbrush or a tack rag to brush off the dust from sanding.
Start painting over the primer with an angled sash brush, painting the corners of the raised panels first. Work from the top panels down to the lowest. Don't use too much paint or let the paint puddle.
Use a 4-inch roller to apply the paint to the raised panels, rolling with the grain of the wood. Remember the door will need several coats.
The rails are the door boards that come across and the stiles go up and down. They meet with a diagonal seam at the corners of the door. Don't paint across that seam. Instead, paint the stiles with their grain, which will be up and down, and the rails on their grain, which will be side to side, and stop the brush stroke (and the roller) at the seam.
Paint the door with several coats, continuing to use the angle brush first and then the roller. The darker the color, the more coats that will be needed.