What size and material do you plan on your goal being? Although it’s more expensive, 8 x 24 soccer goals made of metal are far more safer and stable than one you could make on your own out of wood. Unless you know how to weld extremely well, I advise not to make goals this large of your own.
But you can easily make a compact soccer goal. These are very simple to make out of wood, and if properly sealed, can last many years left outside in the weather.
What are the intended uses of your goal? Do you simply want to have fun with it by yourself or use it for a team practice? Team practice goals will probably take more abuse.
Another creative option is to get a rebounding soccer goal. These things are great for forcing players to follow up their shots.
If you really want to make soccer goals of your own, it is very simple. You simply need a 3-piece front frame, for the top and sides. Then you need supports in the rear, which can be done by forming a right triangle out of wood on each side of the goal.
Whatever you choose, you’ll be sure to improve your soccer goal shooting training easily and have fun doing it.