A Short Illustrated Guide
Unlike in the guides about building a shed foundations and building a shed roof, this guide is applicable to building a door for all kind of sheds - not only rectangular. The door should preferably be rectangular however unless you want to make rounded top corners.
Take measures of the door opening in the shed. You should be pretty precise - if the door is larger than the opening, you won't be able to mount it at all or at least you won't be able to close it. This can be fixed easier than the case of building a smaller door however. So in short, be precise!
Your door can be up to 0.5" shorter and lower than the opening (that's for allowance), but no more than that.
Get 6 - 12 (or more) timber boards depending on the door width. The boards must be high enough so you use only one board on each verticale.
Prime the timber before assembling.
Arrange the boards to fit the width of the door that you measured (i.e. the door opening). If required, cut the first and the last one, but it's much better if you can have all boards with the same size.
If there are gaps between any of the boards plane them well until they fit.
Connect the boards with ledges and nails as shown.
The ledges should be at least as thick as the boards
If required you can add glue or resin between the boards for better firmness and insulation.
Try to keep the ledges on equal distance from each side.
Usually you will want them to stay on the inner side of the door.
Now add two diagonal braces to make the door stable.
If possible use the same material for the braces as the one used for the ledges.
Note how are the nails positioned: be careful not to put many nails on the same thread of the timber, because that can cleave the brace.
It's always possible to use glue but only to add additional strength to the construction.
Take a closer look on how the braces are attached:
There is a triangle riffle in each ledge and the sloped end of the brace fits exactly there.
This will ensure maximum stability of the construction.
Of course you need to make these cuts before attacthing the braces to the door.
In the last step you need to attach the hinges to the door.
In most cases 3 metal hinges will be enough for a stable shed door.
When attaching the door to the shed use tick screws to hold the hinges.
If the shed wall is not stable enough you need to add a vertical beam and attach the hinges to it instead.
Attach the door and you are ready!
(As suggested by Jules Richardson here) the hinges should be on the other side. So I fixed the image in step 6. Thanks!
Do you want to get started? I recommend you to check these shed plans, pick a model you like and go ahead!