Here is what we do(until the stupid trash can sprung a leak):
Fill the can to the union(20 gallons) or bottom of PVC spout(10 gallons) with ro/di water
Add salt, Open red valve close hose valve, turn on pump(MAG 5), add heater if needed
Wait 24 hrs, Siphone out proper amount of water out of tank, wheel can to tank, attach garden hose to hose valve, turn on pump, open hose valve, close red valve, fill tank
Done!
BTW depending on what is in the tank 10 gallons isn't too much. We change 10 gallons a week on our 30 gallon seahorse tank.