Review the United States Postal Service guidelines for mailboxes, including those specific to your local post office. You can find these guidelines on the USPS website. The USPS has specific rules for how tall a mailbox can be and how far away from the road it should be. If your mailbox is not located within these parameters, your mail carrier may refuse to deliver to it and you will have to remove it and reinstall it.
Purchase your mailbox and mount it on a 4-inch by 4-inch by 6-foot wooden post per the instructions provided with the mailbox in regards to proper installation. Various mailboxes have different attachment requirements to fasten them to posts.
Dig a post hole that is approximately 2 to 2 ½ feet deep, approximately a foot away from the curb. Make sure the front of your mailbox will be no more than six inches away from the curb or roadside and just under four feet tall when the bottom of the post is resting on the bottom of your hole.
Insert the mailbox post in the hole and fill the hole back up with dirt. If you do not feel like your dirt will be strong enough to support the mailbox, you can fill the hole back up with any kind of quick drying cement available for purchase at most home improvement stores.
Once the hole is filled in, the mailbox should be securely installed.