If you can make mittens you already know how to make these. I used Lamb's Pride Bulky. 24 stitches (2x2 ribbing) are a nice fit with size 9 needles. Increase every 3 rows, this includes the increase row, to get 8 stitches for the thumb. For Lamb's Pride Worsted 32 stitches is a good fit with size 8 needles, and 12 stitches total for the thumb. I'm about to try 32 stitches on some Noro Kureyon.
I've got another idea for the bootie slippers. Hopefully I can try it out this weekend.