Dig The Hole
When setting any post, always follow the 3-to-1 ratio for determining the depth of your post hole. As an example, a 12-foot post should be set 4 feet deep, an 8-footer about 2 1/2 to 3 feet below ground level.
Dig an 18-inch diameter hole to the appropriate depth for your pole length. When completed, fill the hole with water and allow it time to drain away. This saturates the surrounding soil. Then add 6 to 8 inches of gravel to the bottom of the hole to ensure good drainage. Place a flat rock or chunk of concrete block atop the gravel. Your post will rest on this "foundation."
Mix The Mud
While the water in the hole is draining away, mix the appropriate amount of concrete for your hole's volume in a wheelbarrow. Add water gradually to the concrete, pause and mix it well. Continue adding water and mixing until the concrete attains the uniform consistency of soft pudding -- not too runny and not too stiff. Hand-mixing concrete is an acquired skill. The more posts you set, the better you'll get.
Then, using a shovel, gradually add the concrete to the hole. Add it evenly on all sides of the post, pausing occasionally to wobble the post back and forth. This will help the concrete settle and remove any air pockets. Continue adding concrete until the hole is filled to ground level.
Plumb The Post
Using a level or a two-sided post level, ensure the post is plumb. When you are satisfied it is, attach diagonal bracing props on two sides of the post. This will prevent it from moving while the concrete is curing and hardening.
To speed up your post setting, consider using quick-setting concrete. While more expensive, it cures in less than an hour. It is available at most home improvement and building supply stores.
Before leveling the post into plumb, pre-drill the post where you will be attaching the bracing and use screws when attaching it. Attaching it with a hammer and nails may knock the post back out of plumb.