There are many tutorials online you can check out. Here's one:
http://quiltersworld.typepad.com/my_...ine-patch.html
You can use any size blocks you want. What you do is sew 9 squares into a block, then you cut that block in half down the center in both directions, then rotate a couple of the blocks and resew back into a big block. It's very easy.
I like to use the same color for my center squares in the 9 patches and usually put my darks in the outer corners and lights for the rest. When you reassemble, the center blocks can create a secondary pattern.
As for the math...well you will not get a 12" square, which many people want. I suppose if you wanted to work with odd fractions you could.
4" cut squares will result in a 10" finished block
4-1/2" cut squares will result in a 11-1/2" finished block
5" cut squares will result in a 13" finished block