Building a shed door is a good all-around exercise in general carpentry and design. The door must be square and fit into the opening of the shed. The door should be properly braced with a Z pattern. By building this type of door you will also learn a little bit about general construction principles.
Measure the size and shape of the opening. Simply take a tape measure and read the width and height of your opening. The measurements should be made to the nearest eighth of an inch, and the actual size of the door will be exactly half an inch less than the size of the opening after the doorjamb is installed.
Check to make sure the frame for the opening is square, using a two-foot framing square. If you are building a door to fit an older building, chances are the opening will not be square. In that case, you can build your door to fit an irregular opening or you can reframe the opening so that things are square.
Select the lumber that you are going to use for the door. The wood can be rough cut or finish cut, but it all has to be completely dry, for shrinkage will occur if the wood still has a substantial moisture content. One-inch-thick lumber is recommended, but anything from 1-by-4 to 1-by-8 would be acceptable, and you can vary the width if you like. In fact, a door with different width lumber might be more visually attractive. Lay out your pieces of wood on a pair of sawhorses.
Rip one board with a circular saw in order to come up with the correct width, as chances are that the combined width of your planking will not be the same width as the door opening. This will take a little math and some common sense. Lay the boards out on the sawhorse first, pull them tight together, and then take your overall measurement. You can even use some long pipe clamps to hold all the lumber in place. Subtract the actual size of the door from the larger width; the difference is the exact amount you have to remove from one of the boards. Take the board that you have just ripped and place it in the middle of your design. This will make for a more even-looking final product. Now you have your door cut to width, but before cutting the length, it is a good idea to cut and attach the braces and then fit everything together first.
Cut your braces. For most shed doors you will need three braces: one to go straight across the top, one to go straight across the bottom and a diagonal piece that fits snugly in between. First cut your top and bottom pieces. They will have to be shorter than the width of the door, so that it can be closed properly. If your lumber is 3/4-inch thick, then subtract one inch twice from the overall width of the door. (For example; if your door is 30 inches wide, then your brace will have to be 28 inches long.) If your lumber is 1 inch thick, then subtract 1 1/4 inches twice from the overall width (that will give you a 27 1/2-inch brace for the same width door) and so on.
When you attach the two braces be sure to use 1¼ inch galvanised builder's screws to join the two pieces of wood together. Make sure these two braces are square with the long edge of the door. Once the two braces are attached, your door ought to feel solid.
Lay another plank across the back of the door to complete the "Z" design. You will make two cuts to get this last piece to fit. Hold a straight edge on top of the diagonal board so that it follows the edge of the piece that has already been attached to the back of the door. Now make the mark on the new board with your pencil. Once this is done, you will have to mark the other end of the diagonal piece in the same way. Then make the two cuts. Put the diagonal piece back into place and screw it tightly to the back of the door.
Cut the door to length. You can use a pencil or a chalk line to mark the cut.
Add a gate pull and a barrel bolt above the handle. Instead of a simple handle, you might opt for a gate latch, which will also hold the door tightly in place when it is closed. There are many such items available on the market that will help improve the appearance of the final product.
Using pipe clamps to pull the sides tight is a good way to make your door look more professional; it also helps the structure shed water.
Be careful with the power tools, especially the circular saw. Always do your cutting on top of a good sturdy pair of sawhorses and always wear safety glasses.