Your thumb gusset is made, and the live stitches are on a needle. Thread a tapestry needle and use it to attach your yarn to the left side of the stitches.
Here it's attached. I tied a single knot to keep it in place. The tail end of the yarn goes to the right, and the ball end goes to the left. I tucked the tail end into the inside of the mitten.
Now you want to pick up three stitches. To pick up a stitch, insert a needle as shown below. (The cuff of the mitten is at the top of the picture, and the thumb hole is below the held stitches.)
Then pull the yarn over the top of the needle.
Now pull that yarn through the loop on the needle.
Do this twice more, so you have 3 new stitches on the needle . Pick them up sort of evenly spaced around the thumb hole.
Split the held stitches onto two needles, then knit across those stitches, decreasing 2 stitches evenly. To do this, I did k1, k 2 together, k to last 3 stitches, ssk OR k 2 together, k1.
Now knit around and around until you clear the thumbnail.
Then knit 2 together around until there are 6 stitches left, then cut the yarn and use the tapestry needle to thread through the stitches, tighten, and thread back down into the thumb.
You also get a special bonus blog list. I watched the original Dirty Harry movie today, and it struck me that Dirty Harry Callahan and I have quite a few things in common:
1. unnecessarily hostile
2. good at shooting (he's better, but also fictional)
3. feel underappreciated at work
5. make fun of sociology degrees
6. good at scowling
7. have spent time in San Francisco (only vacation for me, but again, he's fictional)
8. dislike violent criminals
9. don't like to be told what to do
10. get annoyed when lunch is interrupted