Refrain from trueing up the four HSTs. Join a 2.5x4.5" print piece with a matching background piece on one side. Do this for all four units. I used a 12.5" square ruler to transport my pieces between the cutting board, sewing machine and ironing board.
Lay out the center pieces, like a cross, then add the four HSTs.
Sew the three pieces of the top row together, follow with the middle row, then the bottom row. Press the seams. Now, join the three rows together, press the seams, and true up the block to 12.5 inches square.
May your bobbins always be full,