Unroll the landscape fabric the approximate depth of the cavity behind the wall.
Place the edge of the landscape fabric down behind the retaining wall, allowing the edge to fall all the way down to the bottom of the cavity.
Lay the landscape fabric still connected to the roll over the top edge of the retaining wall.
Cut off the landscape fabric with the scissors so that the top edge of the fabric lies on top of the wall.
Repeat the process with landscape fabric along the entire wall to create a barrier on the backside of the wall.
Fill the cavity with gravel and soil. The Lowe’s website recommends placing a 6-inch layer of gravel closest to the wall if your wall is taller than 2 1/2 feet. Pour a layer of gravel about 6 inches wide and 6 inches deep and then add soil to fill in the same depth as the gravel. Continue adding gravel and soil in 6-inch-deep layers to fill the cavity until you fill it completely.
Cut off the extra landscape fabric lying on top of the retaining wall, making the fabric even with the top of the wall.
Cover the fill you poured into the cavity with decorative stone or mulch, if you desire.