Once you're on the league page, click "Draft Central" and then "Edit My Rankings" (it's next to "Pre-Draft Player Rankings"). From there, you can adjust the order of all available players any way you like.

Whether you adjust the rankings or stay with the default, if you're not present during part (or all) of the draft, the system will automatically draft the top player remaining on your list every time it's your turn to pick. You don't have to adjust anything at all if you don't want to do so; it can manage the draft for you if you're absent.