I am doing this tutorial because my friend
diek09 is just learning about making icons and I can't find a good tutorial on layer masks for beginners. This will just cover layer masks - there are lots of other things going on in this icon that I will skim over.
The icon I am using as an example is this one:

from this picture

This is going to be long because I am going to try to carefully explain many techniques to a Photoshop beginner for whom English is not his first language (although he does really, really well!).
( OK - here's how )