Convertible mittens pattern help

I am working on a pair of convertible mittens for my son and I’ve just finished the thumb gusset part of the pattern but now I’m stuck and I’m not sure how to proceed. This is the paragraph that I’m stuck at:

All sizes: Next rnd: Knit.
Next rnd: K21 (25-28) sts. Slip last 8 (10-10) sts onto safety
pin (thumb opening). Knit to end of rnd.
Next rnd: Knit, [B][U][COLOR=“Red”]casting on 2 sts over slipped sts. [/COLOR][/U][/B]24 (28-34) sts.
Next rnd: (Marking rnd). K12 (14-17) [it may be helpful to
draw a length of contrast yarn through the just knitted
12 (14-17) sts]. Place First marker. Knit to end of rnd. Place
Second marker.

I’m not sure what it means. If anyone can give me step by step instructions or clarify this for me or even point me to a video that would be sooooo greatly appreciated!!

It means to cast on in the row right above where you slipped off the thumb sts. It will close up the space where the slipped sts have been taken off. If you were making the smallest size, you ended up with 30 sts. You took off 8 for the thumb, and that left you with 22 sts. That left you with a space. You are now going to cast on 2 sts directly over that space, which will bring your total up to 24. Now you’re knitting the hand of the mitten, and will go back to the slipped 8 sts to work the thumb, later. Use a backward loop cast on, and do it as tightly as possible, because on the next round, you’ll find that those 2 sts have loosened up.