To start you'll need a sleep sac. Pictured is a fleece one in size 18-24 months. But, this would work for any size. Ribbing fabric or any stretchy fabric for the ankles. And, basic sewing supplies.
Step 1: Take baby and place him inside the sleep sac. Mark with a pen where crotch is and let baby roll away and get back to playing. (no photo of this as he was way to wiggle to photograph and mark at the same time!) My pins are on the inside back of the sleeper.
Step 2: Place another pin 3" BELOW where the crotch pen mark is. You can go further down if want to. I wanted there to be plenty of room to grow and be loose enough for any jamies underneath. Then lay the front of the sleeper on top and mark your zipper where your lower pin is. This is where you will sew closed the zipper.
Step 5: Now your going to mark where to cut to make the new legs. I made a rounded top and slightly angled it down the sides. It's 3" from the zipper on the top and 3.5" from the zipper on the bottom. I just eyeballed the rounded-ness.
Step 6: Cut out on your line. Then cut the very bottom so that it's now open. Then turn the sac inside out and sew along your legs as shown below.
Step 9: Sew your ankle ribbing into the leg openings to finish. This was tough to stretch all the way around. But, it did to and it fits great!
Step 10: Turn it right side out and admire your handy work.
I also tried this with another sleep sac. This one had minky on the outside and a layer of jersey on the inside. It worked great with this one too.
He now has room to move around and still stay warm!