I saw this done on a bunch of sites including House of Smiths, they have a great tutorial: here is the
actual link to the frame tutorial. House of Smiths Frame Tutorial
1. first, you will need:
molding (4 pieces cut to fit your frame-you can do this at home depot, they have a cutting center where you can cut it yourself for free)
I first used the wood glue and glued each corner at a time. I would hold it in place while my husband taped it down onto the cardboard also so that it wouldn't move while it dried.
next I put some dry putty on the crease just to cover it up and make it look less noticeable. Other places that I've seen this done, didn't paint it after but I had a hard time covering up the crease and dry putty well enough so I decided to paint it white with spray paint. It also gave it a nice shine.
last, I used liquid nails to put it directly on the mirror. This is definitely a new man job. I held it in place while my husband taped it down and gave it time to dry. We let it dry for about 24 hours before it was complete. I also should mention-I sanded mine down a bit with sand paper. I did some of it before I glued it to the mirror and some of it after, just to give it a distressed look.