Preparing the Foundation
A basketball hoop must be two things above all else: sturdy and level. To make sure yours will be both, give it a solid base. Use the post-hole diggers to create two 12-inch-wide holes, 3 feet deep, and make them 4 feet apart. Align the holes so that the rim side of the backboard is parallel with the line the holes form. Ideally, these holes would be alongside an already-existing concrete slab (a driveway, for example) so the rim will hang over it.
Pour a small amount of water (about a half gallon) into each post hole to soften the surfaces at the bottom (unless the bottom is muddy or wet already). This will help support both the building forms and the posts when it comes time to set the forms.
Make sure the rest of your equipment is near the post holes, as the process of setting the posts goes fairly quickly. All concrete bags, posting forms and posts should be within reach of the holes and readily accessible. Using the level, make sure the holes you dug are on level ground -- this is an important step, as a slope at the foundation of the hoop can make for an uneven apparatus.
Setting the Posts
Place the posting forms in the post holes firmly at the bottom, even digging them into the ground slightly if possible.
Get another person's help to ensure the posts are level vertically and to hold the post in level position while you set the concrete. One post at a time, have your helper stand on the ladder and hold the top of the post as it stands in the post hole. The post should have one of its flat sides facing away from where the backboard will be.
As your helper holds the post in the post hole, be sure that it is level vertically, measuring it on all four sides. Once you adjust the post correctly so that it is level, instruct your partner to hold the post as still as possible.
Pour five bags of the quick-setting concrete into the post holes, starting inside of the posting forms and filling the hole.
Add water to the concrete, filling the rest of the hole. Fast-setting concrete does not require mixing, as the water should soak into it. The concrete should set in 20 to 40 minutes.
Repeat Step 5 for the other post, making sure that the front of it is flush with the front of the first post. When both posts are set, you should have two posts parallel to each other, standing straight up, 13 feet tall from the ground. Let the concrete set fully before going further.
Attaching the Backboard
Assuming you bought a regulation-size backboard--72 inches wide by 42 inches tall--you should have just enough excess post to attach it and make the rim 10 feet off the ground. Screw the four brackets onto the back of the backboard. Two should be fastened low on the backboard, each 24 inches from the exact horizontal middle of the backboard so that they are 48 inches apart (just like the posts).
Two feet above those lower brackets, fasten the other two, also 48 inches apart. This should form four "corners" on the back.
Using a measuring tape, measure out to 10 feet from the ground, then have your helper mark on one of the posts where the backboard needs to be for the rim to be at 10 feet. That mark will be where the lower brackets of the backboard will be screwed in tightly
Fasten the top brackets to the posts as well to complete the process.