Here are two helpful links:
http://quiltbug.com/Articles/on-point.htm
http://quiltbug.com/Articles/on-point.htm
When I did the image in EQ below, you can see the program chops off the outer edges of the sashings (whether you use a separate color for cornerstones or not).
However, that's not how it has to be. It's just an aesthetic choice.
If you do want it chopped, then you would use the finished size of the white blocks to determine the size of the setting and corner triangles.
If you keep them intact, and want your blocks fully framed, then you would take the finished white block size PLUS the finished sashing size to deterime the triangle sizes.
Whatever that measurement is, you would look it up in the links provided above. You should heavily starch the fabric as you'll be dealing with long bias edges. Try not to manhandle the triangles too much. Take care when laying them out to attach to the rows - make sure they are orientated correctly. You won't be the first to attach one of them backwards or upside down. :roll: Mea Culpa. ;-)
I always like to cut the triangles oversized by an inch or two as I like my points to float (personal preference).
When adding the setting triangles, I always sew from the right angle out towards the edge making sure not to stretch the triangle. There will be little dog ears that I clip off before sewing the rows together.
An on-point setting makes for a much more interesting look than a standard grid pattern. Just take it slow and you'll be thrilled with the outcome.
Post pictures when you're done!
And welcome!