To add the cornerstones the way you have them laid out in the picture you would need to remove all your sashing that it looks like you have already sewn on (the light blue water look fabric)
You can opt to recut your sashing to your finished size plus 1/2" for seam allowance or sew two strips together along the long sides. Cut your sashing strips the same length as your unfinished panel. So say the panel is 16 1/2" unfinished then all your sashing strips should be cut to this same length.
Take two cornerstones and sew them to each edge of a top and bottom sashing strip. Take a panel with no sashing, add the new double wide sasing without cornerstones to right and left sides. Then take this panel and sew the sashing with the cornerstones to top and bottom. For the panel block that will be beside, say the right side for sake of argument, sew a cornerstone to only the right side of a top and bottom sash. Sew a plain sash to right side only of panel. Add your top and bottom sash with cornerstone. Now you can connect these two panels and there will be a cornerstone at each corner.
For the two panels underneath you will not put any sashing or cornerstones on the top edge. For the block in the lower left, sew a plain sashing on either side. Then take one sashing strip and sew a cornerstone to each end. Sew the cornerstoned strip to the bottom of the panel with sash on either side. Take your last remaining panel and sew a plain strip to right side only. Take a sashing strip and sew a cornerstone to right edge only. Attach the cornerstoned sash to the bottom only of your remaining panel. Sew these two panels together. You will have two blocks with no sashing on top but sashing on sides, middle and bottom with cornerstones on bottom. Line them up and sew the two bottom panels to your completed two top panels.
Clear as mud?