Instead of using smaller needles as in pattern, I used size 7 needles throughout. Gauge in US7 is 4.75 stitches per inch in stockinette stitch, which is a little off from the 4.5 st/in in the pattern.

Cast on 32 stitches rather than 36. After 3” k1p1 rib, on the first row of stockinette stitch, increased to 38 stitches.

For thumb:
round 1: k2, M1, k to end
even rounds: k
round 2: k2, M1, k1, M1, k to end
round 3: k1, M1, k3, M1, k to end
continued until 15 stitches for thumb, 38 stitches for rest of mitten.