It's not difficult, but it involves some precise steps... and i'm not sure this is the best way to do this "effect".

I tried to do a mini tutorial for you, hoping it'll be helpfull. Some basic operation are not detailed, as i have very few word to explain all these things in english. Maybe Stephanie or someone else could start from this tutorial to write a realy helpfull one.

http://www.essentiel.net/demo/foreignimg/glasstut.jpg