If you can open the box, just stick it with double sided tape and experiment a bit till you find the sound you like, then permanently fix it, in general though, most seem to put it under the lid, near the base side of the bridge
What darryl said.........
Really, just stick it inside the box under the bass side of the bridge then play for a while and see how it sounds. You can then experiment with moving it around inside the box and listen. It may take a while, but it's worth tweaking until it sounds just right.