NOTE: this is a pattern in progress, and there may (or may not) be things missing, please bear with me on this! ;o)
Cut four pattern pieces (measurements coming!) 2 inside and 2 outside.
Sew 2 insides together, and 2 outsides together on the vertical.
Insides and outsides sewn.
Now sew one vertical seam with ins& outs facing each other, and "wrong side" of fabric facing you.
Sew along the long top (horizontal line) and the shorter bottom, leaving one vertical edge open.
More to come! Sorry to leave it unfinished, but I still haven't figured out the best way to get the batting inside without being awfully frustrating.