Set the tow gauge on top of the uprights, and focus on one side of the car. Slide the gauge to that side, until the pin that sticks up from the uprights bottoms out against the side of the slot in the tow gauge. Keep it pressed against that pin and make your adjustments.

When you want to make adjustments to the other side, slide the gauge to the other side of the car until the pin bottoms out at the end of the slot as above.

One thing I have noticed that helps with reading the markings on this gauge is to view the plates from an upward angle. This makes the degree sight lines look shorter, and are easier to line up with the indicator.