I did this very thing a few months ago and it turned out great. I didn't worry about mitering the corners, but purchased the rosette squares for each corner. You can find them already painted white as well as the molding already painted. I glued this all to the mirror with silicon. You have to use painters tape for the top piece until it dries. Silicon does not "set" immediately. I had read to paint the back of the boards with black so they would dissappear into the mirror; tried on a small piece and didn't like the reflection I got, so I suggest white (which is what my frame was) This was a really easy project. Just be sure and measure, measure, measure before you cut your molding! Go for it - I know you will be happy with the results.