You can start making a baby snuggie by first making a baby blanket. I used a 29"x39" piece of flannel and a 29"x39" piece of blizzard fleece. Sew the two pieces of fabric (right sides together) along three sides, like a pillow case. Open it up and roll the edges of the unfinished side in on itself; pin that side. With a zig-zag stitch, go around all edges of the blanket. This will give it a very durable, pillow-case style finish.
Next, make the sleeves. Cut 2- 11"x13" pieces of flannel and 2- 11"x13" pieces of blizzard fleece. Sew one piece of flannel to one piece of fleece, right sides together, along three sides. Open it up, and fold it in half along the length of the sleeve, right sides together. Sew the sleeve shut using a straight stitch. Be patient with your machine--this part deals with shoving thick fabric under your presser foot.
You should, then, have two sleeves measuring 12" long; the top, or "shoulder", will be unfinished. Lay out the sleeves where you want them on the blanket. I totally eyeballed this part. I figure--if I make any mistakes, baby Aden will just grow into it. Then, just above the "shoulder" of the sleeve, cut a line through both layers of your blanket measuring the length of the shoulder.
Sew the sleeves in either by hand or my machine. You can keep the sleeves dangling on the right side of the fabric for this step.Trim any loose strands and you've got yourself a baby snuggie. Now, just make a matching adult-sized one and you'll be good to go!