From you last post it seems that with the right side facing (the outer side of the mittern) you've already knit the 20sts. They should be on the right needle and the 18sts on the left needle.
"To shape thumb:
Next row - K32, turn, cast on 2 sts"
So with the needles oriented this way, hold the tail of yarn in place with your right hand and pick up the 4sts along the base of the thumb and then continue to knit the 18sts on the left needle.
Glad you've got the thumb sorted!