Mount the BoxTopper on the tripod just as you would a camera. Place its threaded socket onto the tripod stud, then tighten the tripod stud firmly but do not overtighten it.
If you use a quick release plate, mount the plate onto the BoxTopper in the same manner, then secure it to the tripod head.

The BoxTopper should be oriented so that when you are standing behind the tripod to take pictures, either side 1 or side 2 is closest to you, depending on the camera you will be mounting. In general, a camera which is longer than it is wide will be mounted with its lens on side 3 and back toward side 1 (you). A camera wider than it is long will have its lens on side 4 and its back toward side 2. There may be exceptions to this with some cameras, but no matter what, either side 1 or side 2 of the BoxTopper board will be closest to you as you stand behind the tripod. This puts the bubble level conveniently at your right or left, as shown below.

Once mounted, make sure the BoxTopper is close to level, then tighten the tripod adjustments so the head is locked firmly into position, not loose.