For the last few days I’ve been having a rather challenging time explaining how to make a mitten thumb via email. Here’s some pictures to go along with my words. Hopefully they will help my friend Shawna, and others working on the same issue.
To make a mitten fit your hand, you need to make it larger on the thumb side. The regular increases create what is called a thumb gusset.
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image source http://www.jessica-tromp.nl
Here’s an interesting photo tutorial: I’ve got a hard time photographing my thumb gussets since I knit in the round, on DPNs. I don’t know that I’d ever knit a mitten on two needles, but it’s neat to know that it can be done.

Once the gusset is long enough and wide enough, I always get someone to try on the mittens to check, I put the gusset stitches on a scrap of yarn and leave them until the hand of the mitten is done–I finish by knitting the thumb.

So, the gusset stitches are on a scrap of yarn, and you are ready to knit the hand. My general pattern is to cast on several (note: this will change depending on gauge) stitches to finish off the thumb hole on the upper side. To cast on these stitches, it’s probably easiest to turn the mitten over (so the wrong side is facing) and cast on, then turn back again.

When it finally is time to knit the thumb, you will use the stitches from the scrap of yarn, the stitches that you just cast on (pick them up). This would make a nice mitten, but over the years that I’ve been knitting mittens I’ve had a pet peeve about holes appearing where the thumb meets the hand. To avoid this issue, I pick up an extra stitch from a row back between the gusset stitches and the cast on stitches on either side of the thumb.

Here’s a view of me trying it on for size. Click to make bigger.
thumb stitch count = gusset stitches + cast on stitches
or
thumb stitch count = gusset stitches + cast on stitches + 2
There are, of course, many ways to knit a thumb. I’m eager to learn more. What’s your favourite method?
P.S. Yes, this is mitten number 2 for my secret santa. It’s been slow going. Things like sleeping, cleaning, singing and holiday preparations have gotten in my way a bit. I am here still, enjoying my holidays. Thanks so much for the comments of concern!