Remove any unwanted trim with a flat-head screwdriver, chisel or pry-bar. Work carefully to avoid putting gouges in the wood surface of the door. Use a screwdriver or drill to remove doorknobs and hinges.
Determine the dimensions for your table by measuring the area in which the table will go and the purpose it will serve. Use a circular saw to cut the door to reflect your measurements.
Sand the surface of the door with rough-grit sandpaper (100-140 grit) to remove surface imperfections. If the door has paint on it, you can sand the paint off or use a chemical remover. Wipe away sawdust with a tack cloth.
Fill any holes with wood putty for a smooth finish and follow up with another sanding with fine-grit sandpaper (150-180 grit). This process may be skipped if you desire a rustic look. Wipe away dust and use a very fine-grit sandpaper (220-280 grit) to smooth the entire piece.
Prepare table legs with sandpaper if they are not in good condition. They should have the same final texture as the tabletop.
Turn the door over, exposing its underside. Measure the location of your table legs and mark the table. Mark the desired depth of the drill bit by wrapping masking tape around the bit to avoid drilling through the surface of the table.
Attach the legs. Depending on the hardware you selected, you may have to drill holes for bracket screws. The easiest way to attach legs is to screw wooden blocks atop the legs and then screw the blocks into the bottom of table.
Return the table to its standing position to check for sturdiness. Adjust as necessary. Decorate and finish according to your concept sketches.