cut the yellow blocks to 6-1/2 inches. the size of the finished triangle blocks, which you will need 8 of for the whole block need to be3-1/2" unfinished. I like to piece my half square triangles larger then trim down, so i would likely cut 4-1/2" squares,mark a line diagonally, sew 1/4" to each side of this line, then press and cut down. is that clear as mud?