This mitten had 17 thumb gusset stitches placed on waste yarn. I placed them on three dpn's as follows:
needle-one: two stitches on each side of the gap
needle-two: six stitches
needle-three: seven stitches
Introduce the joining yarn on needle-one such that the tail (about 7-8") is in the back and the source yarn is in the front. This will cause the tail to be inside the mitten once the thumb is completed. Knit the first two stitches.
Pick up and knit a total of three (3) stitches within the gap in this manner: Insert the working needle into both loops, wrap the source yarn around needle back-to-front, and pull it through. Knit last two stitches. 7 stitches on needle-one.
Knit across needle-two and needle-three. You now have 20 stitches arranged 7/6/7. As you begin knitting round 2, hold the tail yarn tautly so as to snug-up the first stitches. Continue knitting in the round until the thumb is the desired length.
On the first decrease round, k2tog at the end of needles one and three such that 18 stitches remain (6/6/6.) Knit one round.
Finish decreasing in whatever manner you prefer. This is how I decrease from this point:
k4, k2together (three times) = 15 stitches
knit next round
k3, k2together (three times) = 12 stitches
knit next round
k2, k2together (three times) = 9 stitches
Cut working yarn and thread through remaining stitches, pulling tightly and fastening inside of thumb.
Weave in the joining yarn tail (inside.)
This is a close-up of the "gap" area. What I like about picking up three stitches here is the fabric continues smoothly up the thumb.
Here's a side view of the thumb from needle three.
And needle two. No gaps. (But the joining tail is inside the mitten in case any corrective weaving is needed.)