Lots of ideas.
Hi Mar: Along with Williams ideas, you can get some moulding that has a recessed back and use that as your frame. You can either mitre the corners or make buttends. Good luck
Hi there, you could try this:https://www.youtube.com/watch?v=dU9cgAKKoSE
I framed a large mirror in my basement, but the technique should work for a bathroom as well.