Just add water conditioner before you start filling.
The most important part is this.
When the bottom part is open, water flows through, creating suction on the left part, which is connected to your aquarium, which will drain it. If the tank is higher than the sink, you can turn the water off once you have siphon, but I leave it on to get a bit more force. If the tank is lower than the sink you must leave the water running. I doubt it waste too much water on smaller tanks.
Then to fill it up, you just close the bottom, and water has nowhere to go but iinto the sink. If you wanted to, you could use a thermometer to measure the water temp, to try to get it close before turning filling the tank, but I just guess.