How often and how much water am I supposed to be changing out.
That all depends on what you are keeping or plan to keep. Many choose monthly water changes around 20% and others go with weekly changes of 10%. Then there are the exceptions like Randy who have dialed in a system which allows for optimal stability.
Regardless of how much you change, curing your fresh salt water for at least 12 (ideally 24 or more) hours is highly advisable.
Some practices I employ
Adding salt to the container first, then diluting with RODI.
Adding the water rapidly to agitate the salt and continuing to stir it until all of the salt dissolves.
If the salt settles out on the bottom of the container then the water won't cure out evenly and getting the salinity on will take forever.
Adding a heater if water or ambient temps are low to encourage everything to dissolve. I remove the heater once everything has dissolved to save energy.
Using a salinity calibration fluid (aka standard) every time when making up some salt water to ensure that my refractometer is reading accurately.
Covering new water once the salinity is 35ppt and retesting before doing a water change.
Finally, ensuring that the total dissolved solids(TDS) on my source RODI water is minimal, ideally 0ppm TDS.