Here's how I make them, and I'm uber lazy...
Get two pieces of fleece, regular width and 1.5 to 2.5 yards long, depending upon the height of the intended user. I usually go with a pattern on the outside and a solid on the inside, but that's just me. The one I'm making next is for me, it has flamingos on the outside and hot pink on the in. Then get two zippers- heavy duty ones. One is long- 36" to 48" while the other is short- 18" works well. Make sure you get the kind that open at the top and the bottom- they're called "separating sport zippers" if you get Coats brand.
Pin the pieces of fleece together, face to face. (solid color fleece doesn't usually have a "right side" and "wrong side" but sometimes patterned fleece does)
Sew three of the sides together, leaving one of the short ends open to turn the thing right side out. Trim off any extra selvage or excess fleece before you turn it though, or it'll get bunchy.
Turn it right side out and make sure the corners are poked all the way out. Lay it open with the inside facing up.
On the closed short end position the small zipper so that the end is at the outside corners. Pin. On the sides, position the long zipper so that half is on each edge, starting a few inches from the bottom and running up. Pin. (note- if you're making a sack for a child, you may not have enough room to start a few inches from the bottom, so feel free to start the zipper at the bottom corners)
Turn the OPEN short edges inside and pin shut.
Sew all the way around all 4 edges, catching the short pinned edge so that it's closed, as well as all the zippers.
When you're finished- try it out! Stand up and bring the short-zippered edges over your shoulders, like a cape. Zip shut. Bring the long-zippered edges around your lower body and zip shut. There's a place for your arms and hands to stick out and a place for your feet to stick out so you can walk around. When you sit, the sack should be long enough to cover your feet.
If you are a talented sewer, you can catch the zippers in the first step of sewing- when you first sandwich the pieces of fleece together. This will make for a more "professional" looking snug sack, but I've never had a complaint by doing it the other way!
