Begin by preparing your workspace. Lay a drop cloth or heavy towel inside the shower stall to protect the base from scratches. Gather all the necessary tools and the new drain kit. Although you are not working with water supply lines, it's good practice to know where your main water shutoff is. Ensure the area is well-lit and ventilated.

The first step in removal is to take off the strainer or drain cover. Most strainers are attached with one or two screws. Use a screwdriver to remove these screws and lift the cover off. If there are no visible screws, the cover may be a snap-in type; try to pry it up gently with a flathead screwdriver or putty knife. Be careful not to drop the screws down the drain.

This is often the most challenging step without access from below. You will need a special tool called a shower drain wrench or a 'Drain Key'. This tool has expandable arms or prongs that fit into the crosshairs or lugs inside the drain body. Insert the tool into the drain, engage it securely, and attach a wrench or pliers to the top of the tool. Turn counter-clockwise to unscrew the drain body from the threaded pipe fitting below. This may require significant force, especially if the drain is old. Once loosened, you can unthread it by hand and lift it out.

Once the old drain is removed, you must thoroughly clean the drain opening. Use a putty knife to carefully scrape away all the old plumber's putty, silicone, and any other residue from the top surface of the shower base around the hole. Then, use a rag to clean the surface where the new drain will sit. If needed, use a mild solvent like mineral spirits or denatured alcohol to remove stubborn residue. Also, clean the inside threads of the drain pipe below as best you can. Temporarily stuff a rag into the drain pipe to block sewer gases and prevent debris from falling in.

Remove the temporary rag from the drain pipe. Your new drain kit is likely a 'no-caulk' or compression style drain. The main components are the drain body (with a threaded lower section), a rubber compression gasket, and a strainer. Some drains may also come with a friction washer (a thin plastic or cardboard ring). Refer to the manufacturer's instructions. Some drains specify using 100% silicone sealant on the gasket, while others rely on compression alone. For the top flange that sits on the shower base, you will use plumber's putty.

Take a generous amount of plumber's putty and roll it between your hands to form a rope about 1/2-inch thick. Apply this rope of putty to the underside of the new drain's top flange. Place the drain body into the opening in the shower base and press down firmly and evenly. This will cause excess putty to squeeze out around the edges, which is normal. Do not wipe away the excess putty yet.

With the drain body pressed into place, slide the large rubber gasket onto the threaded tailpiece from underneath the shower base. Ensure the tapered side of the gasket faces up, making contact with the underside of the shower base. Next, slide the friction washer (if included) up against the rubber gasket. Finally, thread the large locking nut onto the drain body. Hand-tighten the nut as much as possible. To fully tighten it, use the shower drain wrench or another suitable tool to hold the drain body stationary from above, while tightening the locking nut from below using channel-lock pliers. If you have no access from below, you must use a drain that tightens from above (like a WingTite drain), where tightening screws on the flange deploys a sealing mechanism underneath. For a standard drain, you must tighten the nut from below until it's snug, compressing the rubber gasket to create the seal.

Once the drain body is fully tightened, use a putty knife or your finger to wipe away the excess plumber's putty that squeezed out around the flange. Screw the new strainer cover into place. The installation is now complete.

The final and most important step is to test for leaks. Use a drain plug or stuff a rag tightly into the drain opening to block it. Fill the shower base with 1-2 inches of water, enough to completely submerge the new drain flange. Use a marker or a piece of tape to mark the water level. Let it sit for at least 30 minutes, then check if the water level has dropped. If it hasn't, you have a good seal on top. Now, remove the plug and let all the water drain. If you have any way to check the area below the shower (even a small inspection mirror through a vent), do so now to check for any drips from the gasket seal below. Wait 24 hours before using the shower to allow any silicone or putty to fully cure.