You add salt (preferably marine tank salt) in sufficient quantities to bring the specific gravity of your current water to around 1.004-1.010. You will need to do this very slowly, though....over the course of 48-72 hours would be best.
What worked for me was 1 cup Ocean Salt to 5 gallons of water. Remember though when you top off the tank due to evaporation, just add water since the salt remains and does not evaporate off.