Cut 2 - 9" squares, one of each of your fabrics. Layer the 2 fabrics, right-sides together. Using a fabric marker, mark a grid on the back side of 1 of the fabrics, marking at 3" intervals both horizontally and vertically.
(I stitched with contrasting thread so you could see the stitching easily.
IF you'd like to make a different size HST, the basic formula is:
size HST (2.5") X 3.6 = size of fabric squares (9")
Remember to round up if you like a little more leeway. And the size of your beginning fabric squares needs to be divisible by 3. Note that other sizes were not actually tested.
By cutting a 9" WOF of my 2 fabrics, and then cutting squares from there, I was able to get 4 sets - or 72 HSTs - in no time at all. And I don't see any reason you couldn't expand on this idea and make even more. Yay.