Drill a small hole through the retaining wall with a masonry bit. You want to install the helical tiebacks about halfway up the wall.
Excavate the soil outside the hole, so you can place the bearing plate against the hole.
Place the screw anchor in the opening you drilled in the wall. Use a drill with a high-torque motor to drive the screw anchor system into the soil. Follow the manufacturer's recommendations for depth and torque.
Remove the anchor end of the screw, which is on the face of the wall, and replace it with a smooth metal plate that comes with the helical tieback system.
Repeat the process down the length of the wall. Grip-Tite, which manufactures earth-anchoring products, recommends placing helical tiebacks every 5 feet.