Installing the Spike
Measure from the top of your post and cut it so that the bottom of your mailbox will be 36-inches high, using a circular saw. Save the cut-off piece to use as a pounding block for installing the spike.
Select the spot you want your mailbox installed. Position the point of the spike on the spot, with the bracket aligned so that the post will sit square. Fit the offcut piece of four-by-four into the bracket and drive the spike into the ground with a sledge hammer about six inches.
Set a level on the off-cut post piece and lean the spike as needed to level it. Check it front-to-back and side-to-side. Drive the spike in the rest of the way, until the bottom of the spike is completely flush with the ground.
Installing the Post
Fit the mailbox post into the bracket. Mark the bolt holes through the bracket onto the post. Remove the post and drill 1/4-inch holes, three inches deep. Fit the post back into the bracket.
Drive one three-inch-by-5/16-inch lag bolt through each hole in the bracket into the mailbox post using a socket wrench. Tighten the bolts until the head presses firmly against the face of the bracket. Repeat for each bolt.
Tamp the dirt around the stake bracket by dropping the head of the sledge hammer onto the ground, packing it down.