You're supposed to start with a pair of cheapo flat shoes. I had these Crocs sandals that I never wore, so I decided to cut off the straps and use these for the bottom.
Then you find an old sweater - in my case I found a nice, thick wool sweater at Goodwill and washed it to shrink it and make the wool even more dense.
Cut off the arms.
Then put the arms onto your leg with the rounded part on top of your foot. You can pull the arms a little to tighten the fit around your ankle and then pin and sew to make it more narrow. Cut off the excess after sewing.
Then the fun part! Get out your glue gun and glue the sweater arm to the shoe! Think this through before you start gluing. You will glue it inside out and then turn the whole thing to the right side. You want the shoe part inside the slipper and the sole on the outside. Line up the back seam with the back of the shoe. After turning it right side out, see if you need to add a little more glue in any weak spots.
I sewed a cuff at the top from the back of the sweater. Look how cute!!
There is a problem with these, though. They flop around too much at the feet. The original post did not talk about cutting the top of the shoe off like I did. Looks as if you need the actual shoe to give the sole the structure it needs. I made a tie to cinch the ankle, but it still isn't enough. I'm going to find another shoe and redo the bottom.