If your water is shut off while repairs are being done, there has been a natural disaster, or your plumbing is not working, you still want to be able to flush the toilet. There will be enough water in the tank for one flush after the water is shut off; after that you will need to add water manually to provide flush power.
Fill buckets with water in advance if you know your water will be shut off. If you have no advance warning, you will need to get water from a neighbor or other nearby source.
Lift off the lid to the toilet tank. Set it carefully to one side.
Pour water slowly from the bucket into the toilet tank until the water level is 1/2 inch below the top of the overflow tube. This is the tall, hollow tube sticking up near the flush valve.
Flush toilet as needed. You will need to refill the tank after each flush until the water is restored to your home.