Begin by gathering all necessary tools and supplies. Lay down a protective towel inside the bathtub to prevent scratches and to catch any dropped screws. Ensure your new overflow gasket is the correct size and shape for your tub's drain assembly. Having everything organized and within reach will make the process smoother and more efficient.

Using the appropriate screwdriver (usually a Phillips or flathead), carefully loosen and remove the two screws holding the overflow cover plate to the tub wall. Turn the screws counter-clockwise. Be prepared to apply steady pressure, as these screws can sometimes be tight or have minor corrosion. Once the screws are out, gently pull the cover plate away from the tub.

The old gasket is located behind the bathtub wall, pressed between the tub and the overflow drain pipe. It might come off with the cover plate, or it may be stuck to the tub or the pipe. Use a flathead screwdriver or a putty knife to gently pry the old gasket out of the opening. If it's brittle, it might break into pieces; use needle-nose pliers to pull out any remaining fragments. Ensure the entire old gasket is removed.

A clean surface is essential for a good seal. Use a rag dampened with an all-purpose cleaner or mineral spirits to thoroughly clean the back of the overflow cover plate and the area on the bathtub wall where the gasket sits. Remove all soap scum, mineral deposits, and any residue from the old gasket.

Take the new overflow gasket and position it correctly. Most overflow gaskets are tapered or beveled. The thicker, wider side of the gasket should face the bathtub wall, and the thinner, tapered side should face into the drain opening, towards the overflow pipe. This wedge shape is designed to create a compression seal when the cover plate is tightened. Press it into place around the opening.

Align the overflow cover plate with the screw holes in the overflow drain assembly. Insert the screws and tighten them by hand first to ensure they are not cross-threaded. Then, use your screwdriver to tighten them snugly. Alternate between the two screws, tightening each one a little at a time to ensure even pressure. Stop when the plate is flush and secure against the tub wall. Do not overtighten.

The final and most important step is to test your work. Close the main tub drain and fill the bathtub with water. Continue filling until the water level rises above the bottom of the overflow cover plate, causing water to flow into the overflow drain. While the water is draining through the overflow, use a flashlight to look for any drips or moisture around the outside of the cover plate. If you do not have access behind the tub, check carefully around the plate on the inside of the tub. If you do have an access panel, open it and check for leaks on the overflow pipe itself.

If your leak test fails and the leak persists, the issue may be with the connection of the pipe to the drain assembly itself, not just the gasket. Since you do not have an access panel, you will need to create one in the drywall on the wall behind the tub's plumbing. Use a stud finder to locate the studs on either side of the plumbing. Use a level and pencil to draw a rectangular access panel (e.g., 12x12 inches). Carefully cut along the lines using a drywall saw or utility knife. This will give you the necessary access to inspect, tighten, or repair the plumbing connections.

After the underlying plumbing leak is fully resolved, you must repair the access hole. You can either reinstall the piece you cut out by adding wood backing strips inside the wall for support, or use a commercial drywall patch kit. Apply drywall joint compound over the seams using a drywall knife, embedding drywall tape in the first coat. Apply one or two more thin coats of compound, allowing each to dry and sanding smooth between coats with a sanding sponge. Once the surface is perfectly smooth and level with the wall, prime and paint it to match the surrounding wall.