This process is just for trades that happen DURING the Draft. It only applies to that specific draft, not every draft you do with that league. If you want to input any pre-draft trades, just follow the instructions here: Editing Teams and Draft Order. Picks traded in that manner will apply league wide, and to every draft you do in your league.
First, go to Draft Settings at the top left on the Desktop and in the Gears Icon on Mobile. Then Click 'Trade Players and/or Picks.'
Then select the Teams you want from the Team 1 and Team 2 dropdown. Then, simply click or tap on the pick/player names that are being traded for each team, and click Trade.