Do not force a stuck valve, as it could break. Try applying a penetrating lubricant and gently wiggling it, or use a wrench if needed. If it still won't turn, you may need to turn off the main water supply to your house instead.

After turning the valve clockwise as far as it goes, flush the toilet a few times. If the tank doesn't refill at all after flushing, the water supply is successfully shut off.

Flushing will remove most of the water, but a significant amount will always remain below the flush valve. Using a cup to scoop out this remaining water ensures a much drier workspace and prevents spills when you start working on the toilet.

If you don't have a cup, an old sponge, turkey baster, or even a small plastic container can work effectively to remove the remaining water from the tank. The goal is to get it as dry as possible.