Adding a frame edge to your bathroom mirror adds a lot to a small bathroom. It really improves the space.
Trim - long enough to go around your mirror
Decorative squares - unless you want to cut your pieces at a perfect 45˚ angle
Liquid nails - or gorilla glue
Cut your pieces of trim so they fit the inside of the mirror. Make sure it all fits with the corner pieces and everything. Once you are sure they fit well, stain them. It may need a second coat of stain, if so, do it.
Follow the instructions on the back of the glue package and start placing your pieces. I would recommend starting with the bottom piece, because you won't have any wiggle room with that piece later. Then build up from there, not forgetting the corners. As you go (it's helpful to have two people) use the painter's tape as clamps to secure the pieces of wood. Leave the tape on overnight, checking the frame occasionally to make sure that no pieces have slipped.
Super easy. Absolutely worth the time and effort.