1. Prepare your water. Check its salinity and temperature. Make it match your tank as closely as possible.
2. Look at your tank: a) locate specimens and fish you don't want to get near with a siphon when you suck water out. b). Find a rock that doesn't have specimens on it, that will let you put a hose down on it when it comes to adding water in.
3. Line up your buckets. Get a clear working area and path to the sink.
4. Turn off pumps and lights. Set up a working light from the side so you can aim. This will also drive most fish down in the water: desirable.
5. Put your hose into the tank: if it curls on you, rubberband some unused chopsticks around it to keep it straight. Suck at it to start siphon action. DOn't swallow. Draw out as much water as you have new water to replace. Watch out for inquisitive fish.
6. DO NOT DISCARD OLD WATER YET: accidents happen.
7. use a small lifting pump to suck water out of the bucket via hose into your tank. If you have a strong pump, put the hose end down on the rock you earlier picked as indestructible. This will prevent kicking up sand and detritus.
8. Mismeasured? That's why you didn't throw the other water out yet.
9. Good? Now you can throw the water out and start the pumps and lights again. All done.
10. Test your water in all the usual ways. Wait 24 hours and test again. OK? Mission accomplished.