When making salt water gargle is it best to use plain water or water thats been boiled first then cooled or warm ? can you pre make it in mason jar or any jar and leave set out and use as needed over a few days or is it better to make new each time?

I never go to this level of effort. If your water supply is city water approved for human use I take it directly from the tap, make a briny solution that looks cloudy and use it. It is important to gargle & spit it all out as there is too much salt in it for you to swallow. If you have an uncertain water source do whatever you for daily consumption.