It's automatic. But you should go into Draft Central and arrange players, if you don't like how the computer has them set. You do this before the draft starts and then it will pick in order of what you've got set, assuming of course no one else has your guy.
E. g. your list is
and you pick third. If Joe's not taken, he'll be automatically selected for you. If he is, then you get Fred and of course if they're both gone, you get Steve. And you can do this rearranging even if Milton is #1 according to Yahoo.