Mods, sizing, and clarifications:

The bottom of the cuff is knitted sideways, hence the provisional cast-on and 3-needle cast off. You can see the cuff bottom for the second mitten, before its 3-needle cast off, in the bottom picture.

I used my hand for measurement as these are a gift. My hand is 7.5” measured just above the thumb. I used the US 3 needles for the cuff, up until the final increase row, where I switched to the US 2 needles. The fit seems perfect.

The picot bind-off flares out badly. I fixed this by decreasing 2 stitches on each side of the mitt on every plain knit row of the trim at the top. I had 36 stitches by the time I did the picot bind off.

I was a bit confused by the instructions for the picot bind off. After you make the two stitches, you bind them off, one by one, and then bind off two more. Then you start the process again by putting the stitch from the right-hand needle onto the left-hand needle and making two new stitches.

I used size 0 needles for the final rows of the thumb because with the size 2 needles, the purl rows made the trim look like a spare tire around my thumb. It’s just right when knitted with the 0 needles. The thumb itself is done with size 2 needles, like the rest of the stripey part of the mitten and the trim at the top.

I’m using the traveling jogless stripe technique described here.