The thumb attachment:
Work the palm side of the mitt, work the pre-knitted thumb, work back hand side. That’s the whole round. The same principle as in raglan sleeves on a sweater :) Why this way you ask. Cause, this way you can knit til you run out of yarn and you don’t have to pick up stitches afterwards, when you’re “almost ready”. This way when you’re ready you ARE ready :D

Enjoy!