You can sew the sashing on just like you do for horizontal or vertical rows but you lay out your sashed blocks diagonally & add a setting triangle onto each end. Half Squares on the top/bottom & sides. Quarter Squares on the corners. There are instructions on the internet for figuring out the size to cut the triangles but I usually measure mine and do it the old fashioned way each time I diagonally set a quilt. It's easier for me than looking for the instructions each time--LOL.
You can also lay out the blocks diagonally then sew in rows like normal with or without sashing.
The setting triangles give you lots of space to show off quilting designs.
I'm sure the math whizzes on this board can help you figure out the triangles once you decide on your block & sashing sizes.