Make a brush? "make" I say

Ok I've read up on how to install brushes and use them etc. I've looked closely at all the tutorials that I thought could help me.....Creating brushes, etc. Even found one that said I can make them on PS Elements, which is what I have. But I didn't find anything on how to actually "make" one. As in from the beginning.........new file, blah, blah blah. I need specifics. ~laughs~ For instance........Can you use a screen cap to make a brush? Can you make them in other sizes besides 100 x100? Does anyone have a step by step program.....er....tutorial on how to do this? I need some hand holding please.

BTW, I recommend that everyone look through the archived tutorials. I learned some really cool stuff and saved some tuts in my favorite places so I could go right to them.