Use a shovel to level an area for the fire pit. Most square fire pits are about 36 inches to 44 inches square. Shovel out any uneven areas, and use a hand tramper to make the dirt compact and level.
Dig a trench around the perimeter of the fire pit. The first course of blocks will be placed inside this trench. Use a spade to dig a trench about 12 inches deep, with straight sides framing the fire pit.
Fill the trench with 6 inches of drainage gravel. Spread the gravel evenly.
Lay the first course of brick on top of the gravel. Pack the bricks tightly, making sure the seams between bricks are tight. Use a rubber mallet to tap them into place if necessary. Check the first course with a level.
Use a caulk gun to apply masonry adhesive to the tops of each block. Lay the second course of blocks directly on top of the first course, tapping each block lightly with the mallet to secure it into place. Offset the blocks so the first course's seams are in the middle of the blocks above.
Fill the center of the fire pit with 6 inches of drainage gravel. Spread the gravel until it is even. Compact it using a hand tramper.
Lay the third and fourth courses, using masonry adhesive in the same way you applied it for the second course of blocks. Offset each course of blocks to provide more stability to the fire pit.