This mission is very simple. Sonny Forelli doesn't like the fact that you aren't giving him a cut of your operation in Vice City. He has dispatched some Mafia goons to "collect" from each of your businesses.
The first business the collectors will go after is the Boatyard, so make your way there to try to head them off. When you get there, kill both of them. (They ride around on Sanchez dirtbikes.)
After they are both dead, more will continue where they left off. If you got to the boatyard quick enough to prevent them from "taxing" it, head towards Downtown on the main road until you come across the next group. Kill both of them as well, and you will complete the mission. At this point, the final story mission of the game will be unlocked.