Casting on in middle of project?

There’s 2 things that confuse me, first of all, I’ve just finished the thumb gusset, and now it’s saying I must knit right through it… don’t I cast some stitches on in order to make a hole for the thumb? and secondly, how can I knit 12 - 24 stitches… either number is not to the end of the mitten, it would stop before reaching the end

[COLOR=“SeaGreen”]K12 sts; remove markers and place the 9 sts on stitch holder or strand of yarn - check[/COLOR]; [COLOR=“Red”]k12 — 24 sts. Cont in St st until piece measures approx 5 inches from beg ??[/COLOR]. End on a WS row, placing a marker between two center sts

any help or suggestions?
this is my first mitten :aww:

The instructions mean to knit the remaining 12 stitches of your mitten and you will have worked 24 stitches total minus the 9 stitches for the thumb.

When you go onto the next round or row you will by pass those 9 stitches you have on the holder forming the hole for the mitten wearer’s thumb.

So do I cast on an addition 9 stitches to get around to make a hole for the thumb?

No, you do not need to cast on an additional nine stitches. Depending on your pattern you should either complete the knitting of the body of the mitten (if you are knitting in the round you will simply tug a little bit when skipping over the thumb stitches) or if you are knitting the mittens flat the pattern will either instruct you to knit and sew up the thumb and then complete the two sides of the mitten separately or vice versa.

The Gifted Mittens by Kate Gilbert have an excellent picture of what a flat mitten might look like while working on the thumb and letting the palm/hand portions of the mittens hang out.