Surprisingly simple to do... you gain the ability to liberate slaves immediately after the first mission of the DLC (if I remember right). From there you can run around that town liberating 3-5 in random encounters, but your best bet are plantations. Each plantation gives you up to 30 or 40 (depending on your story progress) slaves. Just be patient with them. There are three plantations on the first island and then another couple throughout the map. The other big hall are slave ships, unlocked after the 3rd or 4th mission. They have NINETY slaves on them. makes that 500 go by real fast.
The first thing I did was go around the map exploring each area and getting the treasure chests, secrets, and view points. After that you should be around 300 slaves. Another couple plantations (they re-spawn pretty quickly) and maybe another slave ship or two, and you're good to go!