Move the lawnmower onto level ground and turn it off to prevent the lawnmower from moving if you accidentally press down on the pedal.
Place the clamp on the bottom of the pedal extender over the top of the mower pedal. The clamp might be a little larger or smaller than the pedal. Center the clamp, as much as possible, over the pedal. As long as the clamp can be tightened, any difference in size should not matter.
Tighten all of the nuts on the clamp to secure the pedal in place with a wrench. On some clamps, it might be easier to hold the nut in place with the wrench and tighten the connected bolt with a ratchet.
Check each nut a second time for tightness; however, do not over-tighten the nuts. The clamp nuts are tight enough if they cannot be easily moved by hand.
Adjust the height of the pedal extender until it is comfortable to reach it while seated on the mower. The method for adjusting the height varies based on the design of the extenders. On some models, you need to tighten or loosen a bolt that adjusts the height of the extender directly. In other models, you need to remove the bolts that hold the two halves of the pedal extender together and reattach them at a different setting, similar to adjusting the height on a basketball hoop.