 When installing any door window sensor, first locate the raised guides on one side of the sensor which will need to match up to the raised arrow on the magnet when installed. The gap between the raised markings on the sensor and the raised arrow on the magnet should be less than half an inch. Ideally, the sensor should be placed on the fixed frame of the door and the magnet should be installed on the moving door. If this is not possible due to the size of the frame, it is most important to mount the sensor flush on the surface. Best practice is to leave a minimum of a fourth an inch gap between the magnet or sensor on the door and the edge of the door frame. Often doors are hung out of square or the vinyl, wood, or metal parts can swell due to weather conditions. Leaving a space will ensure that the magnet does not contact any part of the frame when opening the door, which could cause the magnet or sensor to be knocked off. Make sure to subscribe to our YouTube page and click the Show More tab underneath the video where you can view valuable links pertaining to this product, similar how-to videos, and our low-cost, no-contract alarm monitoring services.