These are less like the 80's old school legwarmers, and more like a very long footless stocking. Warm and cozy, like a sweater for your legs, thicker than socks and more convenient than tights for wearing all the time, since there's two of them, so bathroom breaks are more comfortable.
I'll preface this project by saying that I don't know whether this will work for everyone. I may just have odd proportions, but I'm sure it will work for most with a little fiddling to get the desired fit. This sweater fit me snugly as a sweater, and worked without much adjustment like this, so those proportions work on my body, at least.
I had this old sweater that I really wasn't too comfortable wearing anymore and a pair of cold shins. I knew they were somehow meant to be together.
Start by cutting the sweater into pieces like this. Two sleeves, cut so the cut end is more or less perpendicular to the folded side, and the bottom cut off around armpit territory.
Separate the front from the back of the bottom half of the sweater. On some sweaters, this will mean cutting the side seam right out, and on others, you'll just have to undo the little "crocheted" seam from the manufacturer.
Fold the front (or back) in half, wrong sides together, and lay the sleeve on top of it, along the folded edge. Cut a triangle of fabric away from the sides, up to the cut edge even with the sleeve width. I made sure to leave the whole of the original ribbed hem intact, since I have sort of thick thighs, and new I'd need all that width. If your legs are thin, you might want this cut angle to be less drastic. Sew these sides together along this edge, keeping the piece folded as is, to create a tube.
Keep this the tube you just made inside out, and the sleeve tube right side out, then slip the sleeve into the other, line up the seams and sew around to make one long tube of them.
And that's it, a legwarmer. The seam isn't attractive, admittedly, but it smooths out when it's on, and they're mostly just for staying warm under my nightgown, so who will know?