A shower drain consists of a base, which the plumber attaches to the drain hole when installing the shower, and a strainer. Once the base is attached, you usually don't have to remove it unless you remodel the shower, but you may have to remove the strainer. This is a plastic part that screws into the base, and it can leak if it cracks or if it isn't properly installed. Water seeping between the strainer and the drain base can drip underneath the shower and cause water damage, so it's important to attach it properly to make the drain watertight.
Clean out all the old putty from inside the shower base. Use your finger to remove loose putty, and a toothbrush or wire brush to scrub it out of the screw threads. Reach in and pull out the old rubber gasket.
Install a new gasket by folding it in half, inserting it into the drain opening and working it into the slot just under the opening with a flat-head screwdriver.
Pack plumber's putty generously under the lip of the strainer to make a continuous bead with no spaces. If you're replacing a broken strainer, be sure the new one has the same type of screw threads as the base. There are two possibilities -- coarse and fine. Take the old strainer to the hardware store when you purchase the replacement to be sure you get the right type.
Screw the new strainer into the drain opening, and tighten it as much as you can by hand. The plumber's putty should begin to ooze out from around the strainer. To tighten it more, insert the tip of a pipe wrench; open the wrench to wedge it against the sides of the strainer and use the handle as a lever to turn the strainer.
Remove all the plumber's putty when the drain is tight, and then attach the grid. Some grids have screws that you can drive with a Phillips screwdriver, while others simply snap on.