You have more than one tutorial right here on this board; in fact here's one I just posted on another thread a couple days ago.
It uses two layers of fabric, any size you want....if you need 18 HSTs, right onto the fabric draw a grid of NINE squares 1" larger than the desired HST finished size (you get 2 HST from each square). In your case, the two fabrics would need to be at least 14" square.
Jan in VA