You'll be using the Keepers menu found when you click the Gear button labeled "Draft", see the graphic below:
6.0 Set Up Keeper League
If you do not have keepers in your league, skip this section of the quick guide and go directly to section 7.0, mock drafting. If you are still reading this, we'll describe how to add keepers to each team in the draft. Click the Gear button and select Keepers. This opens a submenu box called Edit Keepers. See the graphic below.
You can add Keepers to each team in your league using this interface. If your league requires that every owner keep the same number of players, you can add them in as "Before Draft" - in this case, the players will be listed as members of the relevant team before the draft starts. You may have to lower the number of rounds in the draft if your league uses this option - the total number of players on the team with "Before Draft" keepers would be however many keepers + the number of rounds in a draft. Start with Team 1 (first pick in the draft) - using the search box, enter a players' name - everyone named this should pop up on the right hand side of the box. Select the correct player from the corresponding NFL team, and that name will pop over under Team 1 on the left hand side of the box. Continue adding players until all of that team's keepers are listed. Continue doing so for each team in the league - when you have listed all the necessary keepers, click the green Done button at top right to save these changes.
However, if not every team is required to keep the same number of players, you'll have to edit in the kept players as corresponding to a particular draft pick - in this case, the number of rounds in the draft will = the number of roster spots for this league. For example, if owners have to spend a first round pick to keep a player, you list the relevant kept players in that round by searching for their name and then selecting the correct round to assign that player (Add To Round 1). If your league rules say it costs a fourth-round draft pick to keep a player, edit the keepers into that particular round. The interface will accommodate a wide range of keeper rules using the Add To menu. Any kept player is removed from the wider draft pool available to the other owners.