The afterthought thumb may be a way to go, I don't know of any videos, but it's pretty easy. Knit till you get to the thumb sts, pick up some waste yarn and knit those. Transfer them back to the left needle and knit again with your working yarn. Then keep on with the mittens in the round. Take out the scrap yarn and put the sts on your needles and knit the thumb, you may want to pick up one st at each end of the sts where the upper and lower ones meet to avoid gaps, and if that's too many, just dec them on the first round.