You need a tool/ruler like the Tucker Trimmer or Tucker Trimmer II where the diagonal is also marked. Are you sure you have the right measurements? I come up that your finished block will be 5 1/4". You have 3 rows to make the block. Each row, left to right: first row - plain, Ohio star, plain. Second row- Ohio star, plain, Ohio star. 3rd row same as first row. If I am correct, When you sew the 3 sections of each row together, the unfinished row will = 5 3/4" width x 2 1/4" height. After you sew the 3 rows together, the unfinished block will measure 5 3/4" x 5 3/4" (5 1/4" x 5 1/4" finished).