In a top-down mitt, you cast-on in the round and work a tube. At some point, you may want to create a thumb gusset without breaking the yarn. You can do that by casting on more stitches, joining them back to the main tube, then continuing in the round. For reversible lace, I use a waste yarn tab to mimic the tubular cast-on and bind-off I've used elsewhere in the project.
- Knit across the tab,
- yarn over and slip one purlwise with yarn in front for the first pair,
- then work alternating knit 1 in the running thread, slip 1 purlwise with yarn in front across the tab.
- After that, turn
- and work 1×1 ribbing across the tab.
- Now that the tab is established, join to the body of the mitt using a crochet hook to work the last knit-purl pair on the body a second time. This produces a secure join that won't gap.
If you prefer a taller thumb, that is still possible but you'll need to break the yarn. Before you cast on for the mitt, cast-on and work a little tube for the thumb and cut the yarn, leaving enough tail to weave it. When you are ready to insert the thumb gusset, just work across the little thumb tube, join back to the body of the mitt, and continue in the round.