You loosen the screw but do not remove it. Then you wiggle the strap through the opening. By pressing down, you can pass the entire length of the strap through the hole. Then you lift the cap, so that it is centered over the hole, and tighten the screw, bringing the inner and outer parts together for a tight seal. The back of the strap is shaped so that it automatically centers in the hole, although it is designed to do so for a hole that matches a standard commercial size opening. (If you buy ½ inch gaskets, which are best for your situation, the cap probably won’t tighten exactly in the center of the hole.) An off-center fit is fine, but turn on a flashlight in the box. If the lumen is passing through a gap, loosen the screw a little and push the cap to a slightly different position. If you are working at height, try removing the screw and reversing the direction. Then press the strap from inside the box and tighten the screw from the outside.