Take the door off its hinges with a screwdriver and move it to a work area. Remove the knobs, latches and any other hardware, so it's only wood in front of you.
Set up your vibrating sander with rough sandpaper. Go over the whole wood surface of the door, taking off any existing finish and getting down to bare wood.
Reload the sander with medium sandpaper. Repeat the sanding process, smoothing out the wood. Repeat a third time with fine sandpaper, getting the surface completely clean. Wipe off the dust.
Apply wood stain over one section of the door. If door has panels, do one panel; otherwise, do about 2 square feet. Let the stain sit for one minute. Wipe up the residual stain with rags.
Repeat for each section of the door, including the edges. Let all the stain set for 10 hours.
Spray on a light, even coat of polyurethane to the whole door. Let it set six hours.
Sand the polyurethane by hand, very lightly, with the fine sandpaper, just enough to dull the surface. Wipe up the dust.
Spray on a second layer of polyurethane. Let it dry, hand-sand it, then spray on a third layer. Let it dry for 24 hours.
Reassemble the knob and latch and rehang the door.