Glue the switch into the slot in the 3d printed case. Plug in the battery and the Circuit Playground and fit them inside the case as well. Screw the lid onto the case to hold them in place.
The final build is up to you. I used half of a polished Seashell Trinket Box. It already had a gold wire around the edge of the shell and already had a loop for attaching to the 3d printed case.
If your shell doesn't already have an attachment point, there are a lot of ways to attach the pendant front. Drilling holes in the 3d printed case and the shell and running jewelry wire through as a hinge works great.
Here are a few photos of some of the materials I tried. Post photos of yours in the forum!