If you squared the blocks then you can cut all the sashing to the size of the blocks. I believe 1.5" finished would be a good size for a 5" block. Anything larger could be overpowering. I generally sew the sashing to the side pieces. Then I make a row of sashing pieces alternating with corner stones. I press the sashing and corner stones so they nest. I pin the sashing row to the block row, one by one.