For
Thing-A-Day for Feb. 1, I made cute armwarmers from socks that were too tight.
First, get yourself a pair of socks whose tops you like and that are not going to be way too big for your arms.
Now, lop the foots off of them. Leave about a half-inch of fabric below where you want the end of the armwarmers to be, for hemming later. If you have tube socks, you can make them as long as you want (hell, just snip off the toes, for all I care!), but if you have shaped socks, just make sure you're not leaving any shaping or it'll look pretty weird.
Now, de-fuzz the cut edge. You want to do this before hemming or you'll just be sewing in all those little bits of string and fuzz and there'll be no end of it shedding all over your arm.
Turn that puppy inside out, turn down the hem, and sew flat.
Once it's all sewn up and turned right side out again, put it on to get ready to cut the thumbhole.
Using the tip of your scissors, find the spot on your wrist where the bone just below your thumb joint meets the wrist joint (not just the bottom of your thumb, or you'll not have much range of motion--see completed pic above to see what I mean). Snip gently to get your hole started and then cut about an inch-and-a-half or so. Keep in mind this will stretch a lot, so cut less than you think you need, you can always enlargen the hole later. Now, pop your thumb through and--voila!--you have an armwarmer! Repeat for other sock and you're all done!