Zeroing in a crossbow scope is a challenging activity. A crossbow is operated in a similar fashion to a rifle, as far as aiming and shooting goes. However, crossbows are much more sensitive to your shooting technique than a conventional firearm. Before you attempt to zero in your crossbow, you must first become proficient in consistently hitting your target at the spot where you are aiming. Proper shooting technique is essential to accurately use your crossbow.
Set up a target on a back stop suitable to stop a crossbow bolt; for example, layered pads of foam. Do not use hay bales or Styrofoam, since neither one is sufficient to stop a crossbow bolt. Stand at a range of approximately 10 yards from your target.
Load and cock your crossbow, and take aim at your target. Hold your crossbow steady and slowly squeeze the trigger until the bolt releases. Try to maintain your aim throughout the process. Shoot three bolts and check the grouping on your target. If all the bolts are in a group of approximately one inch, then you're ready to start zeroing in your crossbow. Continue to practice until your shooting is sufficiently consistent.
Determine the necessary scope corrections to align your crosshairs with your bolt group. For example, if your group is low and to the right, then adjust your sight up and to the left. Zeroing in your crossbow is a process of trial and error. Continue shooting groups of three bolts and adjusting the scope as needed.
Increase your range to 20 yards and repeat the process. Once you can consistently hit your target at this distance, your scope is zeroed in.