Summarize:

Locate your toilet's shut-off valve, usually near the back of the toilet and connected to the wall. Turn the valve handle clockwise to shut off the water so you can work on the toilet without any spills or leaks. Do not use pliers or wrenches to twist the valve handle, as this can permanently damage it. Open the top of the toilet tank so you can watch it drain as you hold down the lever. Keep the lever held down until the entire toilet tank has drained. Inspect the tank for any puddles on the bottom, and sop them up with your sponge to prevent spills as you handle your toilet. If you have a significant amount of water left in the toilet tank, use a turkey baster as an alternative. There will also be residual water left in the toilet bowl—sop it up with your sponge or turkey baster before working on the toilet. The water supply line is the tube that connects the shut-off valve to the toilet. Use a wrench to unscrew the supply line from the tank so you can move the toilet around as you level it.
Close your toilet's shut-off valve. Flush your toilet and hold down the lever. Use a sponge to absorb any residual water in the tank. Disconnect the water supply line from the tank.