You can down load the pattern from Martha. You enlarge the pattern on a copier to your shoe size according to her scaled directions. These are super easy, I whipped them up in half an hour.
To felt a sweater: you need a sweater that is at least 80% wool. Wash it in hot water & dry it on the a high heat cycle. Voila' a felted sweater that you actually meant to do. If you accidentally felted a sweater, now you can put it to good use instead of throwing it away.
Cut out 2 soles & 4 uppers.
Sew the heel together & the front upper seam together. Since these are felted you don't have to worry about any edge finishes since they won't fray or unravel. I left the seam allowances on the outside of the slipper. I used red thread for a "pop" of color (but you can hardly see it).
The pin you upper (or vamp) on to the sole & sew all around. Again I left my seam allowances on the outside of the slipper.
Finished Slippers for toasty toes.
NOTE: I enlarged my slipper to be a 7 1/2 and used 1/4" seam allowance. They turned out to be a size to big.