These will be the last two pairs of mitts I'll be knitting in a while.
I used a short strand of waste yarn
to knit six stitches, and them pulled them out afterwards, leaving stitches with which I was able to create seamless thumbs.
For the first pair, as you can tell, I ran out of blue yarn part way through. One of the thumbs is still not completely seamless since I stuck my needles into the stitches the wrong way and twisted the stitches.
The second red pair is completely seamless. It's also narrower, having cast on only 32 stitches instead of 36 like my previous ones, and fits much better.