To make a 12.5" (unfinished) block:
From your focus fabric:
cut four - 2.5" x 2.5"
cut twelve - 3" x 3"
From your background fabric:
cut four - 2.5" x 2.5"
cut twelve - 3" x 3"
cut one - 4.5" x 4.5"
(sorry for all the white on white)
Fist we'll make 24 half square triangle blocks. Pair one 3" focus fabric with one 3" background fabric. Mark the diagonal, sew 1/4" on either side of the line, cut and press. Trim to 2.5"
Lay out your block and sew pieces into sets of four:
Sew into rows:
Sew rows together to complete the block.