Sorry I can't explain because the process is too long but here are 2 articles that may help you:
http://www.madehow.com/Volume-2/Salt.html
https://www.mortonsalt.com/salt-production-and-processing/
Answer:
There are three methods used to produce salt: solar, evaporation and rock mining.
Solar Evaporation Method. This is the oldest method of salt production. ...
Rock Salt Mining Method. Morton also uses the second oldest method of producing salt – underground mining. ...
Vacuum Evaporation Method