The cellar in many homes is dark and damp and isn't the place for an expensive, ornate door. You can often recycle an old door for use in the cellar by trimming it to fit, but if you don't have one lying around the house, you can make your own from scrap 1-by-6-inch lumber. It won't be as thick as a conventional door, so you won't be able to hang it with door hinges or install a door knob. Instead, use hardware made for fence gates.
Measure the height and width of the door opening with a tape measure and cut enough 1-by-6-inch lumber with a circular saw to fit across the opening when you lay the planks side-by-side with no gaps between them. Cut the boards 1/2 to 1 inch shorter than the height of the opening, more if the ground under the doorway is uneven. Rip one of the outside boards with a saw to make the door the correct width.
Lay the boards on a flat surface and align them so their tops and bottoms form straight lines. Cut two lengths of 1-by-6-inch cross-braces to lay perpendicularly across them. The lengths of the cross-braces should be the same as the width of the door.
Place the cross-braces perpendicularly on the door planks about a foot from the top and bottom of the door. Screw them to the planks with 1 1/4-inch wood screws, driving at least two screws into each intersection of one of the cross-braces with a plank.
Place a length of 1-by-6-inch lumber long enough to extend from the outside corner of one cross-brace to the opposite outside corner of the other diagonally on top of the cross-braces. Mark the lines made by the inside edges of the cross-braces with a pencil and cut along the lines with a circular saw. Place this diagonal brace between the cross-braces and screw it to the planks, driving at least 2 screws into each intersection with a plank.
Screw gate hinges to the front of the door over the cross-braces with the screws that come with the hinges. Set the door in the jamb high enough so it will clear obstructions on the ground when you open it and screw the hinges to the door casing or the wall.
Cut three lengths of 1-by-2-inch lumber to fit around the inside of the door jamb to make a door stop. Close the door and nail the stop to the jamb with 6d finish nails. It should be set tightly against the closed door.
Screw a gate handle and barrel bolt to the front of the door at convenient heights. Close the door, extend the barrel and fit the barrel retainer over it. Screw the retainer to the door casing or wall. If you have to lock the door, screw a padlock hasp to the outside of the door and the casing or wall and lock the door with a padlock.
Paint the door or finish it with clear finish to protect the wood from the moisture in the cellar. There is a variety of gate latch that you can use in place of a barrel bolt to keep the door closed. Some are keyed so you can omit the padlock.
Wear safety glasses when using a circular saw and keep your hands away from the blade while it is spinning.