Starting a tank? How is "cycling" done?

I have a 55 gallon freshwater tank and I am doing well. Now I wish to add a 75 gallon saltwater tank. I understand the basics, like the things I will need to buy and what fish can or cant live together and all that. But I have no clue how to actually start. Do I buy the "living rock" and just stack it the way I would like? What then? Just googling "how to start a saltwater tank" doesnt really explain things out for me. Any advice is appreciated thanks.