It took four tries but I finally figured out how to assemble the baby sleep sack – this one is actually #2.

I found a pattern online:
http://www.bombshellbling.com/baby-sleep-sack-pattern/
and made one, but the binding was a hassle – after doing two more, I realized that the whole would go together much more easily if I did the following:

• when cutting the pieces, make the front neckline about 1 1/2″ deeper than the back neckline
• sew a six inch seam to join the two fronts together at the bottom (5/8″ seam), press seam open
• attach the zipper (extending the top about an inch above the neckline – (it will get cut off when the facing is applied – be sure the zipper slide is below the cutting point!), press flat.
• sew shoulder seams together, press open
• add neck binding (1/2″ bias tape is the easiest to use for this purpose) – attaching to the wrong side of neckline (leaving 1″ extending beyond the zipper edge, and be sure to have the zipper facing the edge so it will zip properly), press
• turn binding and top stitch a needle width from edge (turn in edges at the zipper edges), press
• turn armhole openings under 1/4″ and stitch and press
• now pin front to back, aline arm holes – because Sarah McKenna cut the front and back the same size the back is a bit wide through the sack – trim excess away, sew a straight seam to secure the pieces, then serge around the bag, turn right side out and press flat
  
• you’re done!

This goes very quickly and is easier to execute than the original instructions. I haven’t bothered to adjust the pattern so the finished front and back are the same size – I will probably do that.