Things You'll Need
Channel locks or pliers
New rubber gasket
Old cloth rag
New cardboard gasket (or friction gasket)
Not all shower drains are constructed the same. Some are made with a shower pan that is tiled; some are constructed using screws; still others are snapped in. Most require some sort of sealant.
Stand in the shower stall. Use a screwdriver to remove any screws holding down the shower strainer. Do not drop those screws down the drain.
Use a set of pliers or channel locks to unlock the body of the strainer. Turn it counter-clockwise to loosen and remove it.
Move the body of the drain with your finger so you can detect the leak that is coming from the rubber gasket. Pull out the rubber gasket to see if it is wet. Check to see if the cardboard gasket has rotted. Clean out any crud that you find at the base of the shower pan and around the rim of the body of the drain so you can replace the rubber gasket.
Remove the old putty from the threads in the body of the strainer using a brass-wired brush. Use a piece of old cloth rag to put in the pipe drain to catch any putty bits that might be loose.
Slide the new cardboard or friction gasket and rubber gasket into the body of the drain and at the base of the shower pan. The cardboard gasket should be set underneath the rubber gasket. Coat the shower strainer with plumber's putty and screw it back into the shower drain.
If the body of the strainer is cracked, repairing this requires more attention than a simple cleaning.
Exclude the friction gasket if you can get a firm grip on the body of the drain while tightening it.