So...I thought I would use my Dad's upcoming birthday as a chance to try it out. I made a pattern of what I thought the top should look like then traced the bottom of one of W's slippers to use as the slipper sole.
I cut one top for each slipper and two bottoms each (since that is the part they always wear through I wanted to make it double thickness).
Sewed them altogether then just slightly turned the ankle edge under (to the wrong side) to finish it off.
Then, even though I had already felted the sweaters before cutting, I threw them in the washer for a bit more felting to sort of tighten them up and finish them off.They came out quite nicely and I'm pleased with them. W tried them on so I could be sure they would fit my Dad as well. It probably only took about an hour total to make them and I think now that I've got it figured out I could make another pair in only half an hour - a big time saver for me compared to when I knit them. We'll see if they pass the test when my Dad tries them out. If so, I'll have to tell him that now he can go ahead and shuffle to his foot's content since I can make slippers so much more quickly now.;-)