Yes, you have to know your gauge.
Measure hand around the knuckles. Multiply the number of inches around the hand by the number of stitches per inch in your gauge. This will be the basic number of stitches for the mitten.
Subtract 10% from your basic number and adjust to make it a multiple of four. Cast on that number of stitches and work in K2P2 rib for the desired length.
Knit one round, increasing to 4 stitches less than the basic number of stitches by throwing in a M1 (cast on one backward loop tightly) as needed, evenly around the hand. Knit a couple more rounds before beginning the thumb gusset.
First increase round: Knit to where you want the thumb to be; it's easier if it isn't at the beginning of a needle. Place a ring marker on your needle and increase one stitch. I like a lifted increase, made by using your right needle tip to lift the stitch below the stitch on your left needle, and knitting into it (don't twist it). Knit 2 sts. Make another increase, this time by placing the tip of the left needle into the stitch below the stitch you have just knitted, and knitting into it. Place another ring marker. Knit to end of round.
Knit 2 rounds.
Increase round: Knit to marker, slide marker, increase one stitch as above, knit to second marker, increase as above, slide marker, knit to end of round.
Continue to increase every third round until the gusset is big enough to accommodate the thumb and is as high as the webbing between the thumb and hand.
Knit to first marker then slide all the gusset stitches onto a piece of waste yarn. Cast on 4 stitches by backward loops (across the thumb gap) and knit to end of round.
Knit even until the mitten is 2 inches shorter than the desired length, then decrease as you would for a sock toe. For a better fit, you can angle each of the thumb gussets toward the palm of the hand by a few stitches. This will give you a left and a right mitten.
Divide the circle onto your needles so there are two halves, for the palm and the back of the hand (my description will assume you are using 5 needles). If you want to angle the thumb, make the division a few stitches to one side of the center of the thumb gusset (remember to reverse this for the second mitten).
Knit to within 3 sts of the end of the first needle, k2tog, k1; k1, ssk, k to end of 2nd needle; k to 3 sts from end of 3rd needle, k2 tog, k1; k1, ssk, k to end of 4th needle.
Knit one round even.
Repeat these two rounds until you have reduced the number of stitches to about a third or a quarter of what you started with--or until the mitten is long enough--and graft the tip closed.
Pick up and knit four stitches in the cast-on stitches at the thumb gusset, place gusset stitches onto needles and knit around, k2 additional stitches (to get you to the center of the picked-up stitches) and place marker. Knit thumb even for about an inch, then decrease one stitch each side of the marker every third round until nearly to the end of the desired thumb length. K2tog around, k one round, pull end of yarn through loops and pull end shut.
Copyright notice: You are welcome to print copies of this pattern for your own use and to make items based on it for any purpose you choose. However, you may not reprint this pattern for any commercial use, or place a copy on a website, without express permission from the author.
Judy Gibson, Descanso, California.