You decide what kind of quilt you want. It is easier to design a block if you know the planned quilt destination or color needs. Pick out your fabric to send to everyone and if you have specific needs like "all yellow" or "all black" so you end up with two-colored blocks, let everyone know in the note you send with your F8 fabric. In the last boom I sent a different fabric to everyone to make a quilt for my eldest granddaughter who prefers dark jungle colors. I included bright colors in the F8 because the jungle is loaded with a variety of colors. I told everyone what I plan on doing with the blocks and then cut the Boomers loose to do whatever their creative desires wanted. I had no color requests or theme requests. I got a bunch of wonderful jungle blocks.
Take a look at the pictures Sewbizgirl has posted to get an idea of the beautiful blocks the boomers have made. Her signature line has the link.
BOOM 29 is shaping up to be an amazing boom, as usual. I will be lurking this time around as I have yet to put any of my boom blocks into a finished quilt.