A flat, plain door may function perfectly well, but it can make a room look as drab and boring as its own surface. Turning even a flat hollow-core door into a classically panelled door (or at least getting it to look like one) is easier than you might think. You can use standard wood trim--pre-finished and arranged in what amounts to a series of picture frames, with mitred corners--to form the panels.
Remove the door from its hinges by tapping them out with a hammer and an old flathead screwdriver. Don't use a good screwdriver, as this procedure can nick it up. Lay the door out a work space, face up.
Stain or paint your trim pieces before you start to make finishing easier.
Mark out where you want the trim to go. Generally, you'll lay out two, four or six squares, created by a perimeter of trim, with the trim edges set 4 to 6 inches in from the door's edge. Make sure you don't put trim where the doorknob is. Layout the pattern with masking tape to be sure you are happy with your design.
Measure the top span of the first marked square. Mark out the measurement to a piece of trim, using two marks and leaving space beyond each.
Lay the trim on a mitre saw, with the top edge against the saw's back rail and one of the marks directly in front of the saw blade. Turn the blade 45 degrees inward, so it's pointing toward the middle of the span. Make the cut.
Move the trim so the other mark is in front of the blade. Turn the blade to point 45 degrees in the other direction (so it's again pointing toward the middle of the span). Cut it so the top edge of the board is wider on both ends than the bottom, and angles inward.
Apply glue over the back of the piece of trim, covering it completely. Press the trim onto the surface of the door, with the longer top edge of the board along the top line you drew and the two ends angling back in. Using the brad nailer, place two nails in the piece to hold it in place while the glue dries.
Repeat the process for each edge of each square on the surface of the door, mitre-cutting the ends of each one. Fit them together on the door with mitre cuts butted tightly at the corners.
Allow the glue to dry for eight hours. Set the door back on its hinges.