This is one of my favorite quick fixes for a bathroom. All you need is a couple of hours and about 20 bucks in supplies. Here's how I do it.
First, measure the sides of your mirror, go to my happy place (Home Depot) and choose your moulding. Use their saw box and cut your 4 side pieces. Cut longer pieces than you'll need to allow for your mitred ends. (I usually add on about 8 inches per piece, which is way more than enough, but it makes me feel confident I won't be short).
Next, and this is a trick I learned the hard way- paint the back side of your moulding. Mirrors are REFLECTIVE! When you adhere it to the glass you will see some of the back side show. It would be a good idea if it were the right color. I don't bother painting the front yet, since I'm sure I'll have to spackle in my corners a bit.
Now for the dangerously fun part. Start cutting your corners. If you have a mitre saw- use it. If not, it is possible to use a mitre box and hand saw. You just might have to sand your cut pieces a little.
I just take my time and do one corner at a time, measuring as precisely as possible. These are just basic 45 degree cuts, just keep track of the direction of your moulding.
I cut and glue one piece at a time. Use a level to get it straight. Line up the corners as best you can.
A tip about glue. I use liquid nails. This is PERMANENT so make sure you like it. Put a nice thick bead of it towards the outer edge on the back of the moulding. Again- the mirror will reflect anything close to the inner edge- so keep it to the outer edge. Make sense?
Give the glue a few minutes to start to set up before placing it on the mirror. This will help it stick quicker and help you avoid holding it in place for too long.
If your edges are perfect, good job! If they're like mine they'll need a bit of spackle and sanding to fill the cracks.
Once all the pieces are up, and the cracks are filled and sanded, it's time to finish painting.
Tape off the mirror and the wall and give it a good couple of coats.
Are you glad you painted the back? You're welcome.