It's one of those things that will make sense eventually. Just knit the 7 stitches with a contrasting yarn, not your working yarn. When they have been knit and are on your right needle move them back to the left needle and knit them again, but this time with the working yarn.
When you're done with the mitten and are ready to do the thumb you'll pull that contrasting yarn out and it'll leave a hole with live stitches. Put those live stitches on your needles and now you'll be knitting just the thumb.
Trust me, it works.