1. Hold onto the assembly. While you're under the sink, take the ball stopper off, which you unscrew in the back.
2. Pull your old stopper out and you replace it with your new stopper.
3. The new stopper has two sides, it has a hollow side and solid side. You want to make sure that the solid side is to the back of the sink assembly when you install it, push it down all the way.
4. Reconnect your ball stopper under your sink by pushing it in and tightening it back up.
5. Once properly installed, adjust the rod arm so the fins are below the drain line.