Hi and welcome to KH and to knitting.
At the same time that you add stitches for the thumb gusset, start the cable pattern on the back of the mitt.
For the CO1, you can pick up the yarn between stitches from the previous row for a mostly invisible increase
When you come to the place where you increased stitches in the previous round, work those stitches in k1p1 rib. So for the first round you increase, p1, increase between the markers. On the next round, k1,p1,k1. After the next increase round, p1,k1,p1,k1,p1 and so on until you have 11sts between the markers.
Since you're working in the round here, you won't have a end of row, just a continuous round.
Very cute pattern. Post a picture of the finished mitts if you'd like.