After 3 rnds (rounds) begin shaping the thumbgusset. Left hand: Knit until 7 sts remain on the second needle, pm (place marker), M1, k4, M1, pm. There are know 6 sts in the gusset. Knit the remaining 3 sts on the 2nd needle and the complete rnd.
The gusset after two increases.
Continue to increase on every 3rd row, after and before the marker - after every increase there are 2 more sts in the gusset.
End two rnds after the 6 th inc.
The left hand gusset after 6 increases.
You can make the inc as you like. I do like the first in this video. You can M1 in both ways, one at the beginning of the gusset and the other one at the end if you want. I use the first one at both sides.
If you are knitting both left and right.
Right hand: Work the thumbgusset at beginning of 3rd dpn: k3, M1, k4, M 1.