I’m knitting these mittens for the Lifeline thumb gusset, not the owl. This thumb is so comfortable.
Cast on 36 +1 sts. with size 5 needles for the first row.
Passover the last stitch and first stitch to connect the circle = 36 sts.
Change to size 4 needles for Row 2 and the rest of the ribbing rows.
Add four stitches evenly spaced on last of 21 rows of k2 p2 ribbing = 40.
With size 5 needles knit four rows of stockinette stitches.
Knit increases as instructed in pattern.
Increase 12 stitches for thumb. Add 2 stitches

Above thumb there are 42 stitches.

Decrease two stitches (over the course of 4-6 rows right where the end of pinky area is (but I’m not talking the thumb) = 40 sts. There will be a slight slant in the top of the mitten. In my opinion the mitten fits better.

TOP Decreases: K2 all across row. Knit one row. K2 all across row. End

Future Modifications: (1) I would do less rows of ribbing - maybe 14 rows. (2) The thumb should be knit on 13 stitches, not 12. (3) Always make sure that the edge of lifeline arc matches up with a knit ribbing column to make the arc look like it sprung from the ribbing column.