I've made several of these but just long enough ago to where I'm not visualizing that step. Let's see (looking at my patt and the mitten)...
After finishing the thumb you'll now resume working the actual hand part of the mitten so you should have the orig 29, then join and 'pick up' 2 st (insert your ndl into 2 sts near the base of the thumb) and then work across. In the next row you K the 2 tog that you picked up which'll snug up the juncture of the thumb to the rest of the mitten. (I looked at mine to see if I could distinguish that step but it's quite obscure. The picked up sts essentially form a transition from one side to the other and then are further reduced when you work the dec.)
Not sure that helped.