I'm all thumbs, really, I have all these thumbs. Six of them, and they didn't fit!, bummer.
I made mittens in January, starting with the thumbs. Each thumb is 3 pieces sewn together then made waterproof with an iron on seam tape.
Shown here is an inside out thumb pulled over a mini ironing form. I am applying the tape with a small iron. That thumb is then sewn into the palm of a mitten, but not these, I sewed them with a seam that was 3/8 and they were supposed to be sewn with 3/16 inch. That made each thumb 3/4 inch too small when it was complete. Back to the cutting table.