The first side I used windex and a squeegee. It scared the crap out of me cause the windex took a while to dry.
After I did that, however, people started saying it was a bad idea.
The second side I didn't use any sort of soap or liquid. I weighted the art and pealed the backing off and squeegeed it on (2-person job). There is a pic near the bottom of
this post and a better explanation.
I liked the second method better, but it was quite stressfull. Screw up and you're screwed.
