Whether you’re setting a post for a deck, playset or a new fence there is a right and wrong way to do that job. The right way may require a bit more work. First, dig a hole three times wider than the width of the post. Dig the hole so that it goes deeper the frost line in your area.  Fill the bottom of the hole with 6 inches of course gravel.  You can now set the post in the hole. Secure the post with a cut 2 by 4our when the post is plumb.  You can now mix the Concrete mix and fill the hole to about 3 inches from the surface. You now back fill or place sod on top of the pour once it has dried.