The first video shows you how to do this thumb with the scrap yarn. The second one shows you how to knit the thumb once you've completed the rest of the mitten or fingerless glove. The proper measurement for average size hands is going to be 2 1/2 inches from the top of the ribbing on the cuff to where you want this thumb slot to be. *Knit a lot of mittens this past winter, so I know this is tried and true. Tested on my adult kid "guinea pigs."*
In the video, Kelly purls back on the scrap yarn row. I just slip them back to the left needle like Suzeeq does.