My league requires owners to forfeit a draft pick for each keeper. How can I set this up?

To change the draft order, go to League » Settings » Edit Draft Order » Manually round-by-round and then select the round to be edited. Draft picks for the round can then be edited. Don't forget to click Save after every round.

To delete a pick, go to the far right and select DELETE PICK next to the pick you want to remove.

You can find the round that each player was drafted in by looking in their transaction history or by looking at past drafts. If you imported rosters manually, then you'd have to track the information separately.