I know this isn't technically a free pattern, but Stitch 'n Bitch Nation has a great basic leg warmer pattern. I don't own the book but I was able to access it from our public library, perhaps you would be able to do that?? Even if you just have limited experience on circular needles, you would be able to do this. No increases or decreases, just knitting round and round and round. The size 'L' has a finished circumference of 17.5" at the upper calf.