The initial mixing of SW
can be done in the tank since there aren't any fish in there. Put water in up to almost half full. Then add the salt according to the container of salt you buy. Then add heater and PH to let it mix. Once the temp and salinity is right, you can add LR
and make your scape. If your adding all base rock, don't need to wait for temp and salinity to come up, you can just put it in. After rocks are in, add the sand. Don't want To add salt to the tank if Sand is already in there cuz it may get mixed in the sand and not dissolve right away, and then at some point down the road dissolve and throw your salinity out of whack.
After the initial fill, you'll need to mix salt and water in a container and then add to tank. Can't mix it in tank once fish are in there. Good luck!!