Well it depends do you mean how they are warned now or back then ?
Now: NHC issues a hurricane warning 36 hours in advance of tropical storm-force winds to give you time to complete your preparations
Back then: not sure on what the steps were but here is information about it—hurricane warning service was set up in the early 1870s from Cuba with the work of Father Benito Vines, who served as director of the Meteorological Observatory of the Royal College of Belén. He established a network of observation sites and developed the first method to forecast tropical cyclone movement,