After marking the holes locations, use the Rear Overflow Box and align it to the circles you just drew on the aquarium. If the rear box aligns with the holes and is not hitting the rear plastic trim on trimmed tanks (make sure to test with the lid on the rear box as well), then you are ready to mount the template and drill. Mount the drilling template on the outside of the aquarium. You can use duct tape to mount the template, although we do recommend using spring clamps to hold it in place. DO NOT use screw type clamps, these can cause too much localized stress and crack your glass!