Sew the assembled three smaller blocks to the larger square. The most important point in the pinning is where the white square is on the outside corners...the two red points meeting here need to line up. Pin these seams together first. Sew, and press open.
Next you will sew the three rows together as shown above.
Sew the center row to the top or bottom row. The most important point is where the red center square meets the seams of the two larger squares.
Then sew the remaining row on. The center square is once again the most important point to line up.
Sew the assembled three smaller blocks to the larger square. The most important point in the pinning is where the white square is on the outside corners...the two red points meeting here need to line up. Pin these seams together first. Sew, and press open.
Next you will sew the three rows together as shown above. 
Sew the center row to the top or bottom row. The most important point is where the red center square meets the seams of the two larger squares. 
Then sew the remaining row on. The center square is once again the most important point to line up. 
