I finally took the time to work the magical trick Elizabeth Zimmermann "unvented" to make thumbs into mittens after the fact.
The great thing about the trick is that you just knit the mittens kind of thoughtlessly, without worrying about a thumb, and only add it once you are finished with the knitting of the body of the mitten. You do not even need to hold a few stitches with a waste yarn! All you need to do the trick are the mittens, scissors, your DPTs, and strong nerves if you are thumb trick virgin!
Place the hand of the recipient in the mitten and ask them to bend their thumb. With one of you DPTS lift a stitch where the thumb bends, roughly in the middle of the joint in order to mark it. The recipient can remove their hand.
Next, with sharp scissors, and after having taken a deep breath, snip the marked stitch.
Next, using a DPT gently pull the cut yarn through, releasing live stitches on the left and on the right of the snipped stitch.
I like to use the DPT, as it exerts less pull on the fabric, and seems to minimize the risk of loosing stitches.
The total number of live stitches you need to release will depend on your gauge, and the size of the hands of the recipient.(I released a total of 10.)
Take a first DPT and insert it through 1/2 of the released stitches, with a second DPT, pick up and knit 3 stitches , place about 1/2 of the remaining live stitches on the same needle, then on the 3rd DPT, place the last of the live stitches, and then pick up and knit 3 stitches.
You are all set to start knitting the thumb, just join the yarn, and knit away!
I hope this will inspire you to try knitting mittens the EZ way!