Step 1: Use a seam ripper (or scissors) to remove each arm from the sweater. Be careful the ends will quickly begin to unravel. Now take the remaining sweater vest and put it away, this could end up being a matching hat.
Step 2: Determine how tight you want the top of the leg warmer, by stretching a piece of elastic around your calf. Be sure to leave an extra inch for sewing it together. Sew a piece of material (mine is left over from hemming curtains)to go around the elastic strip leaving plenty of extra fabric for when it's stretched. Now connect the elastic inside the fabric together followed by the fabric with a hand stitch.
Step 3: Turn the sweater sleeves inside out and hand stitch the piece of covered elastic to the smaller wrist section of the soon to be leg warmer. I made the top stitch pretty thorough then just did large stitches with Xs, it reminds me of a Free People type embellishment.