If your pattern says to cast on, then you simply cast on, even if it's in the middle of the row. Don't worry. There is a method to their madness.... you just might not see it right away.
Of course in this type of circumstance, you may have to use a different cast on method than you usually use, for instance you won't be able to do the long-tail cast on. The easiest cast on here would be the backward loop or maybe the knitted cast on if you feel you need a sturdier edge. I'm sure both methods are included in the videos on this site.
Hope this helped! Good Luck!