This is a fairly simple operation. Just a little glue and go.
For this step, you need the pass body, and the button holder assembly that you just put together.
Apply glue to the back of the disk. Press into place. Nudge around as necessary until it is lined up.
Here we see the button surround attached.
Here's a side view of the pass. Notice that all of the layers of styrene are really starting to stack up, and all of the tiny registration errors are accumulating. Now would be a good time to hit this with some sanding paper, and maybe even some putty if you are so inclined.