This is a sewing optional project my sister and myself came up with and completed, in about 5 minutes. Sorry if the pics are bad..
Take an old pair of pants (preferably ones that aren't too tight at the ankle) and cut off the legs a little higher than you want the Legwarmers to come.
Flip them upside down so the rough hem(the one you just cut) is towards the ground. Try them on.
If they are too loose, you can add a thin piece of elastic to the tops and sew over it.
If they fit well,you can skip this step.
Last, you can hem the bottoms or just leave them alone, like we did.
You now have two legwarmers and some extra to make something else with. Enjoy!