Mark the stacked brick wall on the ground using spray paint or a string tied between wooden stakes. Dig a trench along that line, making it at least 2 inches wider than the bricks themselves to allow for backfilling. This Old House recommends digging the trench deeper than the frost line, which varies by area. Consult local building codes and local landscape specialists for more information on the exact depth.
Pour paver base or leveling sand along the bottom of the trench as the base for the blocks. Add 2 inches of the base material and tamp it down so it is compacted and level.
Lay the first row of bricks in the trench side by side. Use a level to help place the bricks in a level row, both as you install each brick and when the row is completed. To make a guide, place wooden stakes at either end of the wall. Tie a string between the stakes to mark the top edge of that row. Place the bricks so the tops touch the string without pushing it up.
Stack the next row of bricks on top of the first row, offsetting the edges to build a stronger wall. The edges of the bricks in the second row should hit the middle of the bricks below so the edges are staggered in each row instead of having all of the brick edges lining up from top to bottom. Cut the end bricks as necessary to get the offset alignment. To cut the bricks, use a brick chisel and a sledgehammer to make a score mark all around the brick. Position the chisel in the line and hit it with the sledgehammer.
Use the lips that are typically along the back edge of landscaping bricks as your guide for positioning the rows on top of the base row. Slide the bricks forward as you install them so the lip is secure against the lower bricks.
Stack additional layers of bricks to complete the wall. Continue offsetting the edges on each row. Check to ensure each row is level before moving on to the next.
Add a layer of gravel behind the stacked wall to backfill it and allow for drainage. Add gravel one row of blocks at a time with gravel against the wall 2 to 3 inches back. Add dirt behind the gravel if there is still space you need to fill in behind the wall. Work up the wall, adding gravel next to it and dirt behind the gravel up the height of the wall. Tamp down the gravel and dirt to reduce settling.