How to use Black salt depends on what end result you wish to attain:     
 ~ As a protection, sprinkle around your house in the same method as blessed salt.
 ~ To banish evil or remove negativity from your space, scatter a pinch in the space and vocalize or visualize the energy leaving.
 ~ To protect from unwanted people or influences, sprinkle some salt outside your doors while naming the person or entity you do not wish to visit your property. You should also name the reasons behind this for added power.
 ~ To remove a person from your home, carefully sprinkle some of the salt behind them, then sprinkle some salt outside of your main door.