Here's a downloadable and printable pattern. Print each of the 3 pages on letter sized paper, then tape them together to get the whole shebang aligned.
Adjust to fit your measurements until you've got a ringer. On me, floor-to-knees is 22" and halfway around my knee is 7.5". My spats come to just below the knee, but yours can be any size that makes you feel dolled up.