The best way to do a water change is to determine how much water you will be taking out of the tank, and then days before you are going to do the water change, set aside that many gallons in a new trash can or barrel. Make sure the salinity in the trash can is the same as the salinity in your tank. Also, get a cheap heater and powerhead to throw in the trash can as well. Some people skip putting a powerhead in there for water flow, but I would absolutely have a heater. The water you are going to be putting in the tank must be the SAME as the water in your tank. Quick, drastic temperature increase or decrease is the easiest way for a fish to get ich, thus, you want to be sure both temperatures are identical.
Also, what kind of water is it that you will be storing, how long do you plan to store it, and where are you storing it? The only water you add to your tank when it is low (from evaporation) is fresh water (water without salt in it). I would recommend getting RO water to store for top off water, but that's just my opinion. Like in your water change scenario, if you will be storing it for a long time, I would get a heater and powerhead here as well.