Bow hunting requires a steady hand and a reliable sight to ensure you are properly aiming for the distance the deer is at, as arrows will fly in an arcing path, meaning a different aiming height is required for different distances. A common sight mechanism is the three-pin sight, which allows for a hunter to calibrate his sights to three different distances, then use those pins to aim at targets at varying ranges.
Measure a point 20 yards from the target, and shoot with the top pin aligned over the center of the bull's-eye.
Observe the vertical deviation from the center point to your arrows. If the arrows are too high, lower the pin, if too low, raise the pin, until your arrows are consistently around the center point when fired with the top pin over the bull's-eye.
Repeat the calibration for the second pin from 30 yards away from the target.
Repeat the calibration of the bottom pin from 40 yards away from the target.
Estimate the distance to your target when hunting. This is a skill that you will learn through experience, though time on the range can also help you to learn distances. The more accurately you can estimate your distance, the more accurate you can shoot.
Position the appropriate pin over the target. For example, if you estimate a deer to be at 30 yards, position the middle pin where you wish to hit it. If the deer is between distances, you will want to aim with a position between the pins aligned over your target, skewing towards the distance it is closest to accordingly.
Fire your arrow when you think the sight is properly aligned.