The best way to prepare new SW
for a water change is add the appropriate amount of salt to your RO
water and let it mix with a powerhead and heater for 24 hrs. Once the salinity and temp match's the main tank, test the ph
and alkalinity to see if they match. If not then you may need to add a buffer but I highly doubt that will be neccessary.
Once the chemistry of the new water is good, remove the desired amount of old water from the tank and simpley replace with the new. Providing the chem match's you should not have any problems unless switching salt brands. In that case do much smaller water changes over 1-2 months to allow the animals a slow adjustment period.
With the old water in a container, it would be the best time to clean out any equipment that needs it as well. Using the removed tank water is the best way to ensure less loss of bacteria and coralline bleaching as apposed to using unsalinated fresh water. Once your done, it can be flushed.