Most mittens are knit from the cuff up. But knitting from the top down has several advantages: You can try on the mittens for sizing as you go, and you can avoid having to sew the end together. You can use any weight of yarn for this pattern!
To be able to make this mitten, you need to know how to (1) do a provisional cast on, (2) knit, (3) purl, and (4) increase by knitting into the front and back of a stitch.
This pattern gives instructions for double-pointed needles, 1 circular needle, or 2 circular needles. It is a full tutorial, with photos to illustrate the steps.