I use size 3.25mm (US3) needles. You can use the magic loop method or double-point needles. I use aran /worsted weight wool. I knit fairly tight and these mittens will fit lg women's to small men's hands. Because you can keep trying on the mittens, you can easily adjust the size if it seems it is getting really big.
Thumb:
Using a large crochet hook (or my fingers) I make 3 chains. I then thread my needles though the edges of the chain loops so that there are 3 sts on each side = total 6 sts. (see picture) I suppose you could use the eastern Cast On Method too, but I prefer this method.
Rnd 1 through 5: First side: Knit first stitch, using a backward's loop CO 1 st, knit to one stitch before end of side, using a backward's loop CO 1 st, k 1 st. Second side is the same as the first side. Each round you will increase 4 sts.
Rnd 6: knit 12 sts, using a backward's loop CO 1 st, knit 1 st. Repeat for second side.
Next 16 rns: knit. Place sts on a piece of waste yarn.
Mitten Body:
Using a large crochet hook (or my fingers) I make 7 chains. I then thread my needles though the edges of the chain loops so that there are 7 sts on each side = total 14 sts.
Rnd 1 through 12: First side: Knit first stitch, using a backward's loop CO 1 st, knit to one stitch before end of side, using a backward's loop CO 1 st, k 1 st. Second side is the same as the first side. Each round you will increase 4 sts.
Next rnd: you will have 31 sts on each side = total 62 sts. Knit 29 rnds straight.
Insert left thumb: Knit 1 st, *knit 1 st from thumb and 1 st from mitten body together,* rep from * to * once, BO 1 st (Slip the second st from the point of the right needle over the st closest to the point). Do this until 14 thumb sts have been knitted together with mitten sts. Now you need to pick up the rest of the thumb sts so that you have a loop of sts again. Knit to the end on the
rnd. you should still have 62 sts total.
[Insert right thumb: Knit 16 sts, *knit 1 st from thumb and 1 st from mitten
body together,* rep from * to * once, BO 1 st (Slip the second st from the point
of the right needle over the st closest to the point). Do this until 14 thumb
sts have been knitted together with mitten sts. Now you need to pick up the rest
of the thumb sts so that you have a loop of sts again. Knit to the end on the
rnd. you should still have 62 sts total.]
Knit 22 rnds.
Next rnd: (first side) Knit 14 sts, k2tog, knit 14 sts, (second side) Knit 14 sts, k2tog, knit 14 sts. = 60 sts
Rib about 20 rnds. BO in rib pattern. I use a knit 1, purl 1 rib, but do whichever rib you fancy.
I put in a graph on Excel, showing how the mitten grows. I usually knit my mittens in two colors with fancy patterns. Feel free to knit it plain or to make up your own color patterns. You can add color to the graph and see how different patterns work out.