These mittens are knit from the fingertip to the cuff starting with a cast on pattern such as Judy’s Magic Cast On or anything similar with magic loop as done in many sock patterns.
It is then knit in the round up to the thumb where a “Traditional” or “Afterthought” thumb can be chosen, both types are included in the pattern. Some familiarity with those methods is helpful, but can be figured out from the pattern.
The size can be adjusted up or down by adding or subtracting 4 stitches to the count and then adjusting the pattern at the thumb, either decreasing or adding rounds to get down to K2tog at each side of the thumb end.
Worsted yarn has been used in this standard size mitten, but with some changes to yarn size you can get a larger or smaller mitten.

I have included a smaller version on 3 weight yarn and sz 5 needles.