I started with:
Exterior & Interior fabrics, 7"x13"
Right sides together, stitch.
Turn, press, top stitch.
Fold in sides (about 2.5")
Then I attached a button and used elastic for the closure.
Also, everyone keep this tutorial in mind because I really really want to do it one day. Also I am really digging this caftan. I also still realllyyy want one of those convertible infinity dresses. I finished one of these tonight, I'll post pictures of it tomorrow or something.