Each center square needs to have white sashing strips on all four sides of the square. First sew a 1.5" x 4.5" strip onto two opposite sides of the square. Sew with right sides together then open and press. Next, sew the 1.5" x 6.5” strips onto the two remaining sides of the squares and press flat. This quilt uses a quarter inch seam allowance for all seams.
The top/bottom pieces and the left/right pieces of blue and green fabric will all get a 1.5" x 2.5" strip of sashing between them. Sew a piece of sashing to a 2.5" x 5.5” rectangle and another piece of sashing to a 2.5" x 3" rectangle. Press open and attach another matching rectangle of the same size to the other side of the sashing.