Lift the strap over your head and place it so that it lies on your left shoulder.
Hook the shoulder strap to your trimmer. On most string trimmers, the hook is located on the upper part of the shaft, in between the handle and the engine.
Loosen the screw so that the hook slides up and down the shaft.
Slide the screw up and down slowly until you find the position where the trimmer is both balanced and barely touching the ground. This is the position in which you will doing your trimming.
Tighten the screw.
Move the trimmer back and forth, without the engine running, at the level you will be trimming. Adjust the position of the strap hook if the trimmer is still unbalanced.