I was fortunate to not have to wait very long before finding a large mirror at a garage sale for pretty cheap. It wasn't yellow but that was easy to remedy. It had the original price tag still on it for $99.99. It had some scuffs in the paint (which I didn't mind since I'm painting it) so the people were only asking $8. SCORE!
This is how it started out.
The back was completely sealed and I didn't want to rip a beautiful paper backing off to paint the frame. So I improvised.
This is what my mirror looked like after I taped it. Just make sure that while doing this you tape only on the mirror and the cardboard and not onto the frame. You want to get the tape as close to the edge of your mirror as possible. It might even be a good idea to see if your mirror shifts at all in the frame. If it does then use this to your advantage and push it as far as you can away from you and then tape it down on the side closest you. Be sure to turn your mirror as you tape each side. You don't want to lean on your mirror to tape the opposite side and hear that awful crack sound.
Once you feel secure with your taping you can begin painting. I used spray paint. :-)
I let the first coat dry over night and then added another coating. You may need to re-tape some spots before you do. As you can see my tape was peeling up but I was able to just stick it right back down again.
Now if after your second coat your paint looks like it's starting to crack and bubble then don't panic. Mine did this for some reason and I thought I had seriously messed up somehow. I just left it to dry another night and when I came back in the morning it had all flattened back down and looked fine.
This is my finished mirror. I think it's even prettier in person. Although it does need a good wash (yikes look at those streaks!). You maybe able to see in this picture that a little bit of paint did seep through to the mirror. Despite my best efforts I kind of figured this may happen. I'm happy to report though that the paint that got on the glass scrapes of very easily. A little scrap with a razor blade would be the most you should need. Or you could do like I did and just use your nails. ;-)