The game’s liberation missions will be unlocked and available to undertake once you have completed sequence 6D ‘Hostile Negotiations’. During this mission you will actually liberate Central Boston, so you do not need to worry about that one anymore!
For each of the remaining five districts of Boston and New York, you will need to look at the map and identify icons that look like a cross with a diamond in the middle of it. These indicate the locations of one of the liberation tasks. Once enough of the tasks have been completed, the rest of the markers will disappear and you’ll be able to pick up a short, simple mission from your contact in the area.
Completing these contact mission, will allow you to recruit said contact as an assassin afterwards (Yes you only get six in the whole game!). Each of these assassin recruits will also introduce an additional assassin ability that can be selected from the assassin wheel, so they are very useful tasks to undertake!