I used my daughter's AG doll to make two squares long enough to fit and wide enough to fit, doubled (just the width). I sewed them right sides together with the bottom left open. Then folded it in half and stitched the bottom and sides together leaving it open for a few inches at the top (maybe 5 or so... I eyeballed it.) I then took some matching cotton scraps and made a small pillow that I velcroed to the sleeping bag.
The bag is made from 1970's vintage pre-quilted fabric that I bought from Selvedge Shop. Sandra is one of my favorite sources on Etsy for vintage fabric. Go check her out if you love vintage fabric and patterns. The cotton was scraps I had left over from lining the jacket below:
My first commissioned garment made for Hyde Von Hitchcock, lead singer of Absinthe.