You can find them randomly throughout the Caribbean, signified by having phrases below their ship such as "Beware the dread pirate Henry Morgan!" A more efficient manor is to check local taverns and speak to the barmaid. If a top ten pirate is normally in the area nearby, she will tell you what settlement they use as a homebase and you can now track the pirate using your quest tracker.
There are three areas notably for top ten pirate activity:
along the coast of Florida and out to the Bahamas;
near Port Royale to Santo Domingo;
in between Campeche and Santa Catalina -- it could also be described as more south than west of Havana, and more west than south of Port Royale, if that helps.
Visiting the taverns in cities around these areas will likely lead you to a top ten pirate.
I recommend boarding their vessel and besting the pirate in a duel, as they will be carrying a large amount of gold and may have a specialist on board, however you can simply sink their ship instead if you would prefer.
Once you have defeated all of the top ten pirates, this achievement will unlock.
You can add quest locations to your quest tracker. To do this from the sailing screen, simply select the map icon in the top left portion of the screen and choose the appropriate quest. If track is not available, it is because you are trying to select a partial map. Once you have a quest tracked, from the sailing screen, click the magnifying glass in the upper right portion of the screen, which will open a mini-map of sorts. On this mini-map, you will either see a flashing red circle, if your target is nearby, or an arrow pointing in the direction of your target.