Maddy, on one of the crocheting groups that I belong to wrote: Honey, you don't need a pattern for that! A muff is just a rectangle.
First decide just how much warmth you need. Last winter I was in Maine; for that cold a climate, I might consider a double layer (one tube inside another for extra insulation) Here in NC I wouldn't bother with a muff at all, except as a fashion accessory in which case I'd use fun fur or some such fru fru.
1. Shop your stash first for a nice warm soft wool. Animal wools are the warmest. Alpaca is nice, warm and soft but not too too fuzzy.
2. Nothing in your stash? Go to a good LYS and shop the sale bin first. You'll only need 2-3 50gm balls, and LYS's will deeply discount NICE yarns when they're down to such small quantities.
3. Check the ball band for recommended hook size for your yarn (that's a good place to start).
4. Crochet a chain as long as you want the width of your muff.
5. Pick a stitch you like from your favorite stitchionary.
6. Work piece until it's long enough to wrap comfortably around your forearm.
7. Slip stitch bottom row to top row to make your tube.
8. If you like, use a fluffy fru fru yarn to go around the ends of the tube.
4. crochet a chain to go comfortably around your forearm, and join to form ring.
5. pick a stitch you like to work in the round
6. crochet until your tube is the length you like
7. skip this step, your tube is already closed
8. same as above
CROCHETING A LONG CHAIN TO THREAD THROUGH YOUR muff, long enough to go over your shoulder like a purse so you can pull your hands out when needed withhout losing your creation