Usually you start the thumb 'gusset' where it gets larger each round (knitting all the mitten stitches). When you have all the thumb stitches you need, you usually put those on a scrap piece of yarn and finish the hand. Then you go back and finish the thumb.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry