Here's how I'm doing it:
Step 1:
Buy sweaters. I went to Goodwill, and found two very nice cardigans, a red one from the gap, and a grey yoked one from old navy. Choose 100% wool sweaters, it's okay if they have holes in them, or whatever, as long as it's not in a place that you will be using. A cute fair isle pattern or something is nice. You could probably buy wool pants or a wool skirt as well, but they would be a thinner fabric most likely, and therefore you would need to buy soles or something to make them last longer.
Step 2:
Cut off anything that isn't wool: buttons, tags, zippers, etc. You could even deconstruct the sweater at this point, but I didn't bother.
Step 3:
Felt. I put them in a Hot/Cold permanent press cycle with some jeans first and detergent first, but they didn't felt much. Putting them in a hot dryer with jeans didn't do much either. I washed them again on hot/cold on the regular cycle with just a bit of detergent, and two old towels and my crocs. Felted up almost perfectly, so I put them in the dryer on a knits cycle. I had to empty the lint trap about three times though. Crazy piles of lint.
This is how they look after step 3:
Step 4:
Make a pattern. I traced my foot on a piece of paper, and cut it out. I'm not sure what people's feet sizes are, but I'm hoping that if I cut one a little big and one a bit small, it will work. Now I have to decide what I want the top of the slipper to look like: Do i want sides, or just a plain pocket style? how many thicknesses? do I want to incorporate any of the features of the sweater into the slipper?
Step 5: cut out. eek! You'll want several soles, and one or two tops for each slipper. how many depends on how cold the recipients house might be, or how thick the felt is. you don't want a super stiff upper, but you don't want the sole to wear through the first winter either.
I did deconstruct the sweaters at this point, so they would lay flat.
The felted sweaters will not ravel, but will be harder to cut through. I cut out three thicknesses of sole:
and one thickness of the top. I would've liked one more upper thickness, but there wasn't any extra. For the second pair, I was hoping to make use of the yoke design. being a raglan-type sweater, there are four shoulder seams, instead of two, so it will be impossible to cut an upper that doesn't have a seam. I've just had to center the upper between two seams. I think it will be fine.
Step 6: Sew together. I used some yarn to baste the sole layers together so they wouldn't stretch while I sewed them together. Then I used doubled thread and blanket stitch, to sew the slippers together. Then I sewed the upper to the sole, also using the blanket stitch. For the red slippers, I folded over the ribbed edge and sewed a button (taken from the original cardigan) on the outer edge of each slipper. For the grey ones, I folded over the edge as well, but no button.
Finished pictures:
