Primarily, it involves calling attention to a certain thing the person is doing as being sinful, and it is best done (in my opinion) diplomatically, privately, charitably, and with an eye to educate. Also recall it is but one of the seven spiritual works of mercy. If done badly, i.e., in a shrill, accusatory, and condemnatory manner, it may actually make the sinner dig his heels in deeper rather than stop and reconsider his behavior. Be very careful when doing it, and make sure you can do it well before proceeding. And be absolutely certain that you yourself are not giving bad example in the same matter!