Masks?

I was looking for tutorials on masks in the memories, and I can't seem to find one for what I'm trying to do. I've come across a bunch of masks people make but I haven't a clue how to use them. I don't know how to get the icon white where I want it to be, and the base to show through. I'm referring to masks such as these made by sir. I use Photoshop 7. If anyone could help me out or direct me to a tutorial that I might of missed... I would greatly appreciate it!