Draw diagonal line on wrong side of corner square.
Draw another diagonal line to form an X.
Now sub cut the block in half vertically.
And again horizontally. You now have your lines drawn through the diagonal on all four of your squares.
Set the seam with iron and remove excess fabric, center piece only.
Repeat sewing last two squares to each corner. First one...
then the opposite.
Again remove access fabric.
And that completes one square in a square.
43 more to go.