Contact local utility companies to determine where you may safely dig. In the U.S., dialing 8-1-1 connects you to local utility companies who will mark the location of underground cables and pipes.
Choose a spot next to the road. Postal regulations require 6 to 8 inches of clearance between the road edge and the front of the mailbox. Curbside boxes must also be on the mail carrier's right-hand side of the road.
Mark the spot with spray paint.
Dig a hole using the post hole diggers. The post manufacturer should specify the width and depth of the hole. If the dimensions are not specified, make the hole diameter three times the diameter the post, and make the hole depth one-third of the post height. Postal regulations require the bottom of the mailbox to be 41 inches to 45 inches from the ground. Adjust the depth accordingly.
Pour 6 inches of gravel into the hole.
Set the post into the hole. Make sure the post remains level and still meets the height and distance requirements.
Pour dry concrete mix into the hole, stopping 3 inches from the edge.
Pour water into the concrete mix according to the manufacturer's directions.
Let the concrete set for 30 minutes.
Fill the rest of the hole with soil and tamp it down.
Attach the mailbox to the post according to the manufacturer's instructions. Most mailboxes bolt or screw on to the post from the bottom of the box.
Attach mailbox numbers to the mailbox according to the manufacturer's instructions. Most mailbox numbers use adhesive backing to attach.
Contact your local postmaster and mention that you have installed a new mailbox. The postmaster may wish to inspect it.