CO 80 sts (pattern repeat is 16 sts), and I normally do 72 sts for a mitt, so that’s why 80 sts.

improvised thumb gusset
star with 3 sts
m1L and m1R on each side of gusset every other row
put on waste yarn when gusset is 19 sts

a little short, may need to add an inch or two of blue ribbing on the bottom of the mitts. but with long sleeves on they work just fine.