Lay out a garden hose along the path of the circle around the tree. If you are looking for a perfect circle, measure the distance from the tree trunk on all sides to ensure it is equal.
Spray paint along the path of the hose to give you a guideline for digging. Remove the hose.
Dig a trench into the soil wider than that bricks, depending upon if you are putting the bricks in side-by-side pattern or more like a train. If you want the bricks to be even with the soil for ease of mowing the grass, the depth should be equal to the bricks plus 3/4 inch. It is critical that you measure every foot along the way to ensure that the depth of the trench remains constant along the path.
Leave the soil to the side; you will need some later.
Add a 1-inch layer of construction sand to the bottom of the trench. It should be smooth and level across the entire length of the trench. Place a brick into the trench with the edge against the interior wall of the trench. Firmly seat it and check to ensure that its top is just above the top of the soil. The brick will settle a small amount after a few weeks.
Continue to lay the bricks into the trench until the circle is completed. Check every few bricks with a carpenter's level to ensure the bricks are remaining level with the soil slope.
Fill in the space between the bricks with extra sand. Fill the space between the bricks and the outer wall with some of the soil that was removed. Lay grass seed on the dirt and water well.
Remove the grass from inside the ring and fill in with a mulch of your choice. Do not lay the mulch against the tree trunk, but instead leave at least a few inches of separation.