When i was working at my double side BBQ quilt i made some photos step by step. For those working on a double side quilt IT can be useful:)
First the blocks for one side, make rows of them:
Do the same for the other side.
After both side rows are done, make 2 small sandwich, each one from both side rows and the batting.
You have: row 1 (two faces), row 2 (two faces), sashing face a, sashing face b.
This is the result (one side):
A good tip was to iron a 1/4″ border to the sashing face a ( the one sewing by hand) before starting.