Well, it's finally done! We actually bought a smaller mirror to frame instead of trying to attach the trim to the giant one we already had.
Brian, of course, executed the frame-making. Here he is using biscuits to join the mitered pieces together:
Here is the frame all put together. Brian also added heavy-duty L-brackets to secure the joints since it would be holding a huge mirror on the wall.
Brian used some wall mirror brackets to attach the mirror to the frame and then used heavy-duty fasteners and wall hangers to hang it on the wall.
And here it is in the 100% finished bathroom:
We love it and can't wait to do something like that in the master bath (if we ever get to it!).