DSB. I used exactly one skein of Cascade 220, with no leftovers!
I improvised the pattern:
CO 60 sts on size 5 dpns, and double-knit in the round (look up double-knitting -- I'm not enough of an expert to explain it here, but it means you have two layers of knitted fabric) for a couple of inches.
Start increasing for thumb gusset -- first on the inside layer, and then on the outside -- and increase every other round (so, once inside, once outside, and then do a plain round once inside and once outside) until you have added 16 stitches to each layer.
Set thumb sts. aside and reconnect hand stitches to continue in the round.
Keep going until the hand is big enough to fit.
Um, decrease a few rows in there somewhere, and on your last round, do a full-on knit-purl-knit-purl to connect the two layers (so the inside layer won't come out when you pull your hand out of the mitten). I think I ended up with about 12 stitches at the end. It makes a pretty star at the top. Then run the end of the yarn through those last 12 stitches and pull tight. Weave in end.
Return thumb stitches to needles, knit the thumb until it's long enough, do a round of p2tog decreases on the inside layer, then a round of k2tog decreases on the outside layer. Do one row of kpkp like for the top of the mitten, then close off the top of the thumb. Weave in ends, and use the beginning-thumb tail to close up the hole between the thumb and the hand.
If desired, use remaining yarn to crochet a chain, and connect the chain to the wrist of each mitten. String through coat sleeves and pretend you are a four-year-old!
Watch out, because these took me a freakin' month to make. Double-knitting is soooooo sloooooowww.