First, measure the circumference of your muff. This will be measurement one for your cover. Mine measured 20 inches.
Next, measure the length of your muff, plus a little extra for overhang. My desired measurement was 15 inches, though next time I'll make it a little shorter.
Cut a piece of fabric that measures measurement one--with no seam allowance--by measurement two--with a one inch seam allowance on each side. My fabric therefore measured 20x17 inches. You don't want a seam allowance on the circumference so it's nice and tight. You want a larger than usual seam allowance on the length so you can make channels for the ribbon.
Iron the length ends under 1/2 inch, then turn and iron them 1/2 inch again. This will make sewing your channels much easier!
Unfold the seam allowances, fold the muff cover in half, and pin the top edge shut. Stop the pins about 1/2 inch from the fold for the channels. These are the vertical pins in the above picture. You're just going to sew between those two pins so the ribbon has room to escape. I used a small backstitch for that seam.
Once you've sewn the seam, refold the channels at the edges into place, and sew. I used a small whipstitch for this, and went over the opening twice.
Thread the ribbon through the channels.
Stuff the muff into the cover, and tie the ribbons. Make sure you can still fit your hands in :)
And decorate! You can decorate before sewing as well, of course, but my trim was so thick I didn't want it in the seam. I also wanted to play with placement.
And now I'm really looking forward to a second cover :)