cast off question

[amourbliss.livejournal.com]

So, I'm knitting a cell cozy and I originally started with 30 cast ons. Now it's time to cast off, but I only need to do 15. I know to cast off I need to knit 2 and pass the 1st one over the 2nd one and off the needle, but what i don't understand is if this whole process is one cast off or if eaxh knitted stich i make is one cast off. please help me!!