This is mainly for my benefit....
I've looked around for information on how to pick up stitches at the side of a thumb-hole opening and haven't found anything that works better than this:
This is for the simplest thumb-hole opening, an afterthought thumb.
The first step is to pick up stitches from the top and bottom of the opening and then remove the scrap yarn.
(There will be as many stitches on the bottom as were knitted with the scarp yarn. There will be one fewer stitch on top.)
Attach yarn and knit the picked up stitches on the bottom of the thumb opening.
Knit together 2 stitches at the side of the opening (or, in other words, knit the next stitch together with the one above it).
With a second needle, knit the picked up stitches on the top.
Knit together 2 stitches at the side of the opening.
Then proceed to knit in the round.
(You may want to work an ssk as the last st on each needle on the first round.)