Screw the mailbox to the top of the post horizontally.
Attach one end of the two wooden side supports to the mailbox base you just attached, and the other end to the post. Do this on both sides of the base and post. The supports should be at a 45-degree angle.
Choose the location of your mailbox. Standard post office guidelines say that the mailbox should be 8 inches away from the road, so take that into consideration.
Dig a hole with a shovel or a hole digger where you want to put the mailbox. It should be about 2 feet deep and only as wide as necessary.
Place the post in the hole. Fill the hole with gravel 6 inches deep, and level the post. Pour a layer of concrete around the post, and level again.
Fill the rest of the hole with soil. Attach the mailbox to the mailbox base.