Remove any hardware from the door. The old doorknob and any existing hinges will need to be removed. Use a screwdriver to remove the screws. Take apart the doorknob assembly and latch.
Strip the finish off the door. Unless the door is very old and weathered, chances are the old finish is still on, at least in spots. Use either a heat gun or a chemical stripping agent to remove all traces of any old varnish.
Sand down the door. Use a heavy-grit sandpaper to remove the old stain and smooth out the finish on the door. For relief spots or grooves, use steel wool to get in where sandpaper cannot. Once the first sanding is done, go over the entire door with a fine-grit sandpaper.
Fill in the hole left by the old doorknob. Fill in the hole with wood putty matched to the color of the wood. Wait for this to dry and then sand it down so that both sides are even with the rest of the table. Alternatively, you can find a wood plug that matches the color of the door. Place wood glue around the edges of the plug and insert it into the hole. Tap the plug into the hole until it is flush with the rest of the door. Sand the top.
Stain and clear-coat the door. Stain the door to the color of your choosing. Use a fine-grit sandpaper in between coats. Once the door is the right color, sand it again lightly, and apply a coat of polyurethane. Let this dry, and then sand again before applying another coat.
Prepare the table legs. Table legs can be purchased as unfinished wood at a local hardware store. Sand them, stain them and then clear-coat them until they match the finish of the door. Screw the new legs into the bottom of the door.