There are multiple types of distance finders, or range finders, in golf. The most expensive models are GPS devices that can track the course and give you exact distances to fixed points, such as flag sticks, lakes and bunkers. These models are self-explanatory and do all the calculations for you. However, less expensive, monocular-style distance finders require a bit more patience and approximation, and generally only give you the distance to the flag stick. They also assume most flag sticks are the same height. Both types of distance finders are helpful for casual rounds, but are not allowed in amateur or professional tournaments. Follow these steps if you have one of the more basic types.
Shut your less dominant eye and peer through the monocular device as you aim it toward the flag stick.
Line up the base of the fixed object with the bottom set line of the crosshairs in the device.
Follow the top of the flag stick toward the numbered portion of the crosshairs in the device and estimate the distance. Generally, the numbers will require some guess work.