I was in an exchange where each participant bought their own focus fabric. They cut 12" squares from it (larger if you want, but a 12" square usually works for a 12" finished block) and gave one to each of the other participants. Everyone made a block of their choice, incorporating the focus fabric, and returned it to the person who chose the fabric. She then ended up with 12 blocks (including her own), all of which contained her chosen fabric.