It depends. You could sprinkle it around the house, or leave a small bowl out in each room to absorb anything negative. If you use it in a spell it should specify.
If you are planing to get rid of the negative spirit, you can use the salt. You could sprinkle it around, or mix the salt with water and sprinkle/spray the water instead. Be sure to visualize and charge energy beforehand, and specify you want all 'negative spirits gone' this should prevent the harmless ones from being expelled [at least it works for me] then cast a protection spell to keep it out.