I've knit 2 pairs of slippers with Cascade 220 yarn.
I use a U.S. size 10 1/2 (6 1/2mm)instead of the recommended U.S. 13 (9mm). it's less work to felt them that way.
Here you can see the 2 new pair with an old one that is the desired size.
Instead of using energy that is hard on the environment I felt by hand using a bucket and a toilet plunger, new and used only for this purpose. A little bit of mild detergent and some hot water and a lot of Lars energy. I do change the water 2 or 3 times during the process, it still only takes 3 or 4 gallons of hot water instead of however many gallons it takes to fill and run the washing machine 3 or 4 times.
After about 30 minutes one pair is shrinking faster than the other pair. For some reason some colors felt less than others, always do a swatch and test this, unless you're me and just take a chance. So, more work on the one pair.
Finally they're all about the right size and can be stuffed with T-shirts and shaped and left to dry. It usually takes about 3 days for them to dry, I take the T-shirts out after a day.
Around my house the slippers get holes in the bottom after a few months of use so I sew on the leather soles available through Fibre Trends.