Most of us have them in our bathrooms. Those frameless mirrors that are just dying for detail. It's so simple to frame them! Molding, paint, double sided tape and call it done.
For this bathroom, I chose fluted molding and corner blocks (all available at Home Depot/Lowes). Your cuts don't have to be as precise. Don't cut it exactly the width, maybe an 1/2 inch long. Gives you some wiggle room on placement. I painted both black then dry brushed a bronze paint over the top.
IMPORTANT TIP: Paint the back of the molding, too. You will see the reflection in the mirror and it will look bad if you don't.
I did not use mirror adhesive to attach this (it's very light molding on a small mirror). I used 3M heavy duty clear mounting tape (it's thick like foam tape but clear so you won't see it in the reflection.
ANOTHER IMPORTANT TIP: Keep the mounting tape to the outer edge so it won't be seen at all.
I've had this up for at least four years and have never had a problem with is coming off.
If you use heavier molding (i.e., MDF or longer pieces) I would advise using mirror adhesive. I have one bathroom where I need to do this because the mirror is extra long. Will post when it's complete to add any tips on using mirror adhesive.