Such a great mitten pattern!
The yarn is perfect for this pattern.

Followed modifications noted by tatankagirl.

Cuff: k1,p1 for 13 rnds
Knit 7 Rnds in pattern before starting the thumb gusset.
Gusset: as tatankagirl, I also knitted three Rnds between increase rounds.

My thumb: knit 14 rnds before starting the decreases.