Its recommended to let salt dissolve and aerate for 24hrs or overnight prior to use. This gives time for the salt to dissolve properly, pH to stabilize and for the water to become oxygenated.
Mixing is recommended to be done with a small powerhead or utility pump. Also salt will dissolve faster and better in heated such as the same temp as your tank. Too cold and the salt will have a hard time dissolving.
As long as your tank salinity and the salinity of your new water matches then your tanks salinity will not change. If it does, then it did not match.
29g Mixed Reef, 150w 14k Phoenix MH w/ Moonlights, Reef Octopus BH1000 HOB Skimmer, Ecotech Vortech MP10es, ReefKeeper Lite (Plus), PH Probe, Temp Probe, ATO, Carbon/GFO Reactor.