Fabric: fat quarters of light and dark fabrics. Include lots of lights, lots of darks, some yellow, red and some "Uglies" in your selections. This is a great place to use up fabrics you are tried of or want to clear up from your stash. The more you bring, the better; trading will be encouraged. Bring at least 9-12 light and 9-12 dark fat quarters, cut as described below. This is a good starting point.
If you want to precut: cut some 3 1/2" squares and 4" squares from both light and dark fabrics.
(If you have boxes and boxes of 2" watercolor squares, bring them. I'll show you how to use them instead of the larger squares.)