After you end your current round you start the next one but only knit 12 stitches of it. You then turn and slip 1p, purl the remaining 11 stitches that you just knitted, and then purl another 12 stitches for a total of 24. You'll be incorporating 12 stitches from your previous round.
I hope this answers your question, I'm really not certain what's confusing you. Did the video help at all? Once you see what it is you want to do, it should be pretty easy for you. Getting the concept in the first place is the hardest part, at least IME.
You can't knit the left sock first. Ever.