This is how I do my sashing and cornerstones. Let's pretend we're working with a 9" block and 2" sashing. I do the vertical strips first. Cut the sashing 9" X 2 1/2 and do one row of blocks at a time. I don't put sashing on the outside edges of the blocks because you'll have a border there. So take your first block and sew a piece of sashing to the right hand side, press and sew the sashed block to the next block. Continue across the row, adding a piece of sashing to each added block. So now you have long sashed strips.
Now for the horizontal sashing. I make one long piece made up of sashing and cornerstones. Cut the sashing 9 1/2" X 2 1/2" and the cornerstones 2 1/2" X 2 1/2.Now join them in one long strip--sashing, cornerstone, sashing cornerstone..... until you have the length you need. Now join that long strip to the bottom of a previously sashed strip of blocks, making sure you match your cornerstones with the vertical sashing. Press. Then add a row of sashed blocks. Press. Continue on, adding a row of strips between each row of blocks. Trim any fabric off the edges. Hope this makes sense and helps. If you understand this, you can adapt it to any size block you want, Just don't forget to add seam allowances when you measure. If you are doing sashing without cornerstones then just cut your horizontal sashing the length you need to go across.