Begin by completely clearing out the cabinet area beneath the bathroom sink to provide ample room to work. Place a small bucket directly under the drain pipe (P-trap and tailpiece) to catch any residual water that will spill during the process. Lay down several old towels around the bucket to absorb any splashes and protect the cabinet floor from water damage. Using a flashlight or work light will make it much easier to see the plumbing components.

Look under the sink for the vertical metal strip with holes in it (the lift rod strap) that connects to the faucet's lift rod. This strap is connected to a horizontal rod (the pivot rod) via a spring clip. Squeeze the sides of the spring clip together and slide it off the pivot rod, then pull the pivot rod out of the hole in the strap. Next, locate the retaining nut (pivot nut) that holds the pivot rod into the side of the drain tailpiece. Using your hand or a pair of adjustable pliers, turn this nut counter-clockwise to loosen and remove it. As you pull the pivot rod out of the tailpiece, the attached ball will come out; be ready for water to drain into your bucket.

Now that the pivot rod is no longer holding it in place, move back to the top of the sink. The pop-up stopper is free to be removed. Simply grab the top cap of the stopper and lift it straight up and out of the drain opening. If it's been in place for a long time, it may be stuck by soap scum and hair; a slight wiggle as you pull upwards should free it.

Before installing the new stopper, take the opportunity to clean. Use paper towels or a rag to wipe away all the grime, hair, and soap scum from inside the drain flange where the stopper sits. A small, stiff brush can help dislodge stubborn buildup. Once clean, unpack your new pop-up stopper and inspect it to ensure it matches the old one and has the necessary hole in its stem for the pivot rod.

Take the new pop-up stopper and drop it into the sink drain opening from above. It's crucial to orient it correctly. The opening or loop at the bottom of the stopper's plastic or metal stem must be facing toward the back of the sink. This ensures that the pivot rod, when re-inserted from the back, will pass through it.

Return to your position under the sink. Take the horizontal pivot rod and slide it back into its opening on the drain tailpiece. You will need to make sure the end of the rod passes through the hole in the new stopper's stem. You might need to wiggle the stopper from above with one hand while feeding the rod with the other to achieve proper alignment. Once the rod is fully inserted and the pivot ball is seated in the tailpiece, screw the retaining nut back on. Hand-tighten it securely. Finally, re-insert the end of the pivot rod into the lift rod strap and secure it with the spring clip, using the same hole as before or a new one for adjustment.

With everything reassembled, it's time to test your work. Push and pull the lift rod behind the faucet. The stopper should move up and down freely inside the drain. Push the stopper down to the closed position and fill the sink with a few inches of water. Wait a minute to see if the water level drops, confirming the stopper creates a good seal. Next, pop the stopper open and let the water drain. As it drains, use your flashlight and a dry paper towel to check for any leaks around the pivot rod retaining nut under the sink. If you see a drip, gently tighten the nut a tiny bit more. If the stopper doesn't seem to open high enough or seal tightly enough, you can adjust this by moving the spring clip to a different hole on the lift rod strap.