This one is a bit of a grind but will come naturally if you enjoy the game.
I did this by focusing only on Order of Souls missions, but to keep the game fun, I would attempt to do every skull fort that popped on my way as the Stronghold Skull will give you a good boost in reputation with the Order of Souls.
You should also check shipwrecks.
Now, if you want to try and speed this up you can cancel the Order of Souls missions that aren't that great. By that I mean ones that sail you all over the map for one or two captains heads. Try for the ones with 4 on an island, and preferably a small island. With larger islands you have to find out where they are spawning, and they may not be accessible to cannon fire. With small islands you can hit 'em with cannons from almost anywhere.
But make no mistake, this one is a grind.