The town has shut off water due to major water line break. How can I use the water in my water heater to, for example, flush my toilet?
Step one, very important. Shut off the heat (electric supply or gas) to the water heater before draining it. An empty or partially empty water heater can be very quickly destroyed if heat is applied while it's not full of water.
Then drain into a bucket from the drain valve, opening the highest hot water tap in the house to let air in if the heater is below the rest of the plumbing.
If the heater is above other plumbing, you can open the highest hot water fixture and then fill from a lower hot water fixture, or possibly hot or cold water fixture depending on possible check valves, etc.
When water supply is restored, run the hot water taps until all air is purged from the water heater BEFORE restoring power or turning on & re-lighting gas supply to it.
The only real option is to connect a small length of garden hose to the drain valve on the bottom of the tank and use that to put water into a bucket. To make the water flow faster, you can also manually open the temperature/pressure relief valve on the top of the heater while the drain valve is opened.
A toilet requires pressure to fill the tank, so just trying to let the water flow backwards through the cold inlet pipe probably won't do much good. That might allow water to slowly drain out of a sink, but really only if the entire water heater is higher than the faucet (my water heater is in the attic, so I could probably make this work).