I forget exactly where I first saw it, but I sash each block and then put the blocks together. You have to layout your top on paper to see which block gets which sashing, but when you have done a couple, you have it down pat and can do it quickly. You will see that each row starting from the bottom, gets sashed on the bottom and on one side. The one end block gets sashed on both side. The top row gets sashed on the top, the bottom and one side. Only the final corner block gets sashed on all four sides. Then you just sew your blocks into rows and sew your rows to make the top. You are able to cut the strips for your sashing on the length of grain, making them more stable and each one is the correct length for the block.
Charlie