I have a container that holds about 35 gallons of water.I also have five 5 gallon containers that I use.I do a 15 gallon PWC
every other week in my 75 gallon with 20 gallon sump.The day before the water change I place the 35 gallon container beside my aquarium and pour in 15 gallons of RO
water and mix the salt in.I place a heater in and small powerhead and run them in the container until the next day.The next day I use half inch tubing and drain water into the five gallon bucket and dump outside three times from my DT.I start siphon using powerhead that is in DT.During this I have sump turned off.After I double check temp and salinity in big container I use a PH with half inch hose to start pumping the new water into my sump,turning sump back on.Make sure you have the PS turned off until you get the new water in to the proper level in your sump.
This I have found is the easiest way for me.15 gallons out 15 gallons in.
In the past I have left the 35 gallon container in the garage And pumped from there with 75 ft of tubing,but actually have found it is easiest to tote the 5 gallon containers the day before into the den.