Once your leaguemates have selected their keeper, the commissioner will need to set the draft order.
Once the order is set, all keepers will appear underneath the draftboard like this:
Here you can see Justin Jefferson is at the bottom below the draftboard, but still in the column that belongs to the owner.
You can choose not to have any costs for the keeper by leaving that player at the bottom.
You can set the round cost for the team by clicking on one of the tiles in the owner's column and set the keeper there.
Once you set a Keeper to a square, they will appear with a "lock" icon and take up that draft slot.
In this instance, the keeper will still end up on the owner's team, but they'll be forced to drop a player after the draft if they are over the roster limit before they can do make any free agent or waiver adds.
On Sleeper, you are always allowed to draft over the roster limit.