With a RO/DI, you'll hook it up to whatever line you can. Let it run for a few minutes to clear the lines. Make sure you have a place to drain the rejected water. After several minutes start collecting the clean water in a bucket,barrel, etc etc. Then once your desired amount is done, take about 5 gallons and store it for top off, (you need an ATO on your list as well). Whatever is left should be used for your water change. Add your salt, something for circulation, and a heater to heat the water to whatever the recommended temp is. Mix the water for the manufacturer recommended time frame, afterwards it should be clear. Check the parameters and make sure they are what they should be and not TOO different from what your tank runs. Once that's all good, drain the amount of water you will be replacing, replace the water, and top off with RODI until your next change. Sounds like alot, but it really isn't.