Now that we have the gusset and the thumb set-up out of the way, we can work on the hand of the mitten. In the last entry I left the mitten halfway through the round, with the thumb stitches transferred to waste yarn. The next step is to bridge the gap between the two needles where the thumb stitches used to be.
I lost two hand stitches when I made the gusset - the stitches at the end and beginning of the second and third needles, respectively - so I'll need to add those back, along with four more stitches to get the total number of the stitches in the hand up to 48 - the number I calculated to go around my son's hand above the thumb plus ease. The easiest way to do this is use the backwards loop cast on - just cast 6 stitches onto the second needle, and then start knitting from the third needle.
I usually knit the first couple stitches from the third needle directly onto the second needle, to keep a large gap from forming after the backwards loop stitches. When I get to the next round, I use a little extra care to knit the newly cast on stitches without too much space forming between them - backwards loop is not very stable at the start and needs to be handled attentively to keep the tension even. After a few rounds, I can shuffle the stitches around so they're evenly spaced again - now with 12 stitches on each needle.
The hand is now set up and I can whoosh through until I reach the top of the pinky finger (about 3 inches).