You do have to mix the RODI with the salt in a separate dedicated tank or bucket (I've used a plastic kitchen garbage can). Obviously it should be rubbermaid type or glass, never metal. The mixing should be with a pump in the water and mixed for min 24 hrs, 48 hrs is better. Test salinity, preferably using a refractometer, and if not, a high quality glass hydrometer (never plastic swing arm). If you have a heater, you can heat that water to match your tank water, but that's not usually necessary if your room temp is not significantly different than your tank. Just keep in mind you don't want to dump very cold water in to your tank that will change the temp of the tank water. Not sure why you need to add prime. If you are actually using RODI water then dechlorination is not needed. Where are you getting your RODI water?