These were an interesting project! Someone requested them earlier this year to replicate an existing slipper they had.
(They’re (mostly) crochet because the instep is knit.)
(original slipper on the bottom right, the rest I made in various sizes)
They found a pattern that was really close, but not a 100% match, so I knew I’d have to make some changes to match the original slipper. It seemed like an interesting challenge, so I had to try!
It took way longer than I expected. First I managed to crochet too much trying to finish the projects I was working on (fish hats…
) and my hands needed a break. Then I could only work on them a bit at a time, and every crochet stitch is a slip stitch (the smallest crochet stitch). 
Fortunately they really liked them when I finished and delivered them! They even requested more in a different color:
I remembered to take a picture of the inside of a slipper:
~
My slipper making journey for anyone who cares:
The pattern I started with: Crocheted Lapland boots
I found yarn that was close to the original, so I started testing the pattern. It took many tries! 
Starting with the cuff, first my gauge was way off:
Then a little better:
I finally got it pretty close!
(the knitting needle is just there to show the pattern better because the edge likes to curl)
The slipper is mostly crochet, with a knit instep. The pattern wanted it done in reverse stockinette stitch, but I didn’t like how it looked. I ended up using plain garter stitch, which matched the original. Reverse stockinette attempt:
Garter attempt (way better!):
I tried following the pattern for the edges, but the toe didn’t look right (not as rounded as the original slipper):
So I had to use some math (
) to figure out the right numbers of increases and where to put them (based on the original slipper).
The sole at least didn’t need many changes, so I could finally started making slippers! It took me a few tries and a few more modifications to get the sizing right:
-The top slipper is my first attempt mostly using the pattern. It was too small and the toe wasn’t rounded enough.
-The middle left slipper pair is my second attempt made with a K hook (6.5mm). They looked how I wanted, but they were too big.
-The middle right slipper pair is my third attempt made with a J hook (6mm). They’re smaller, but still too big.
-The bottom left is slipper is the original.
-The bottom right slipper is my fourth attempt made with an I hook (5.5mm) and a shorter instep. It finally matches the original!
I finally had a matching slipper, so I typed up my notes so they weren’t a chaotic mess sprawled across too many pages of my crafting notebook. 
Congratulations! Your awesome make is one of this week’s Featured Projects! 