Now that you have finished your cuff, it is time to start working in stockinette. On this very first round, increase 4 stitches, evenly spaced. You have 36 stitches on your needles; that means you'll increase every 9 stitches. I like to keep the increases away from the beginning of the round, so I knit 5, then increase 1, then *knit 9, increase 1* 3 more times, then knit the 4 remaining stitches.
Knit 5 rounds plain.
Now it's time to start the thumb gusset. Some patterns have the thumb pop straight out of the mitten; that's a design that does not fit my hand at all, so I always make a thumb with a gusset. You will need two markers. You can use little squishy plastic or rubber rings (my preference), safety pins, or even a contrasting strand of yarn.
Knit 3, place marker, increase 1, knit 2, increase 1, place marker, knit to end of round.
I do my increases as follows: pick up the bar between 2 stitches, lift it onto the lefthand needle, and twist it to avoid a hole, then knit. You can get very elegant and make left-leaning increases on one side and right-leaning increases on the other side if you wish. Check out a book such as The Vogue Knitting Book or The Knitter's Companion for information on making these paired decreases, or check YouTube for videos.
Knit 1 round plain.
Continue working as established:
round 1: knit to the marker, slip it, increase1 stitch right after that marker, knit over to the second marker, and increase1 stitch right before it.
round 2: knit
On one row you'll be increasing one stitch on each edge of the thumb gusset; on the next row, you just knit all the stitches.
Repeat these two rounds, ending with a plain round, until you have 14 stitches between the markers.
Now it's time to put the thumb stitches aside until later. You will need a sewing-up needle and a length of the same weight yarn in a contrasting color -- 12 inches or so is plenty. Knit the 3 stitches to the first marker. Remove that marker. Thread your contrasting yarn onto the sewing-up needle. Now, pass the sewing-up needle from right to left through all 14 thumb stitches (remove the second marker now). Remove the stitches from the knitting needle, making sure you don't pull the holder yarn all the way through:
(Remember -- you can click on any picture to make it bigger so you can see the details.)
I often tie the ends of the holder yarn together to make sure it stays put unless I've used a very long piece Many people like to use those little metal stitch holders that look like big safety pins, or kilt pins. I find that, because they are rigid, they stretch out the corner stitches and cause trouble later. I can't stop you, of course -- but really, you'll get better results if you avoid those things for thumbs.
Now, cast on 2 stitches using the old backward loop cast-on (probably the first one you learned).
Be careful not to leave a big loop when you join these two stitches to the old stitches:
You're back to a total of 40 stitches now, and you are ready to knit the rest of the hand.
Go to Part 3: After the Thumb Gusset