Hi
I'll try and be brief
If you leave water standing, it may remove the Chlorine but it will not remove Chloromines which are found in most water sources and are harmful to fish.
The simplest way to do a water change is -
1. Turn off heater and filter
2. Siphon a bucket of water from your tank and pour it down the drain.
3. Make a note of the temperature of your tank water.
4. Fill 3/4 of the bucket up with cold water from the tap.
5. Add a kettle of boiled water, and put the kettle back on.
6. Then add more cold or more boiled until the temperature of the water in the bucket is pretty much the same as the tank water.
7. When the temperature is the same, add the correct amount of dechlorinator to the bucket.
8. Stir it a bit.
9. Slowly add the water to your tank.
10. Turn heater and filter back on.
11. Make your Mum a cup of tea with the kettle you put back on at 5.