Installing a cleanout plug in a drain allows a plumber to access the drain with a snake or auger tool. This project is not as complicated as it may first sound. Older style drain plugs consist of a solid piece attached to a chain and are still found on older style plumbing systems.
However, the vast majority of drains in newer homes are designed to be fitted with the newer variety of cleanout plugs. Newer style plugs consist of a rubber washer in between two plates, with a wing nut in between.
Step 1 - Loosen Wing Nut
Make sure you thoroughly dry the area around the drain hole with a towel before installing the cleanout plug. Locate the wing nut on the top of the cleanout plug and loosen it with a pipe wrench. Many models don't have the wing nut, so in this case just remove the top.
Step 2 - Fit Cleanout Plug
Turn the bottom plate of the plug sideways and slide it into the drain hole, keeping the wing nut on top of the drain. Rotate the bottom plate so that it is flush against the drain.
Step 3 - Tighten Wing Nut
The wing nut should be tightened until both plates of the plug are pressed flat against the drain; the washer should also fill any remaining gap.