Since I need to increase the mitten circumference around the base of the thumb, there are two obvious paths. After knitting the finger portion:
- Knit a separate, tip-down thumb and join it at that point, then complete the mitten by decreasing down through the palm to the wrist.
- Cast on extra stitches, and continue working down the palm to the wrist. Work the thumb last by picking up held stitches and knitting to the tip.
- The thumb circumference is about 33% of the hand circumference. Cast on half the stitches on a DPN.(Example: my mitten is 42 stitches around the hand, so the thumb will be 1/3 of that, or 14 sts; I cast on 7).
- Work 1 row of I-cord.
- In the second row of I-cord, K1, then repeat [M1, K1].
- Work I-cord until thumb is desired length.
- With a crochet hook, find the first horizontal bar at the tip, twist it into a loop, then chain up the ladders to close the gap in your tube.
Work a decreasing gusset at the base of the thumb stitches as you work down the palm toward the wrist.
-----Aside from the I-cord thumb, there's not much I like about this. The grafting is a pain, and there are messy holes to sew up at the end. I think working the thumb last is a better plan.