Firstly, make sure that the league is set as a Dynasty. To do this you'll want to go into your General League Settings and verify that the League Type is set to Dynasty.
Next, go under League Settings -> Draft Settings to make the "Available Players to Draft" set as "Rookies Only", as shown below.
Then during the draft, after the 3rd round (or however many rookie rounds you'd like) is complete, pause the draft.
Now, go back to Draft Settings and set the "Available Players to Draft" as "Vets Only", as shown below.
Once everyone is ready, you can resume the draft, and you have now drafted your rookies first.
That is how you can draft rookies first, if you have any questions, please feel free to reach out to us at [email protected].