While it's more common to just buy a pre-made post box from the store that includes the mailbox itself as well as the base and stand, you can actually make your own with several advantages. You can save money by constructing your own support system and imbue your own style on the box by choosing something unique. You can use a barrel, paint can or beer keg for your mailbox, or even an old, vintage mailbox you inherited or found at an antique store.
Saw a 4 x 4 to the desired height for your post box plus 2 feet. For example, if you want a 3-foot tall base, cut the wood to 5 feet tall.
Dig a hole 2 ½ feet deep into the ground with a post hole digger where you want to set-up the post bo. Fill the bottom 6 inches of the hole with gravel.
Put the wood into the hole so it is resting on the gravel and surround the wood with more gravel in a 6-inch layer. Pour cement down the sides of the wood to fill in the gaps in the gravel. Only add enough cement so it comes to the same level as the gravel but no higher.
Check the vertical alignment of the wood with a spirit level to make sure it is straight and allow the cement to cure overnight. Fill the rest of the hole with dirt to cover the cement.
Place the letter box on top of a plank of wood and trace the bottom of the box with a pencil. Cut out the traced shape from the plank to form the base of the post box.
Set the base on top of the wooden post cemented into the ground. The pieces should form an upside down 'L' shape.
Attach the base to the post in the ground using outdoor drywall screws and power drill. These pieces form the stand for the post box.
Place the mailbox on top of the base on the stand and secure the box from the inside. Use more screws inserted through the bottom of the box and into the base below.
Attach a mailbox signalling flag to the side of the box with a galvanised nail and hammer if your box doesn't already have one. Make sure you can easily move the flag up and down but it stays in the correct position when moved.
Install a hinge on the bottom of the box opening if the door is already connected. Attach the door to the hinge so it opens downwards.
Decorate your post box with paint and other details like wooden cut outs or accessories that match the theme of your box.
Go to second hand or salvage stores for used mailboxes that you can paint and treat how you want without spending the money on a brand new box.
Do not place your mailbox without checking the local regulations of the Postal Service.