This mission is a series of three, sneak/fight your way into a restricted area and blow up target barrel mini-missions. Once the mission has begun look at the Havana map. You will see that there are three new restricted areas here now, each of which has a green objective marker associated indicating a gunpowder store that we need to destroy.
The goal at each of these locations is simply to detonate the glowing crates; you can do this by sneaking in using your stealth techniques, or alternatively walk in in full sight, murder all of the enemies in sight and then shoot the barrels as easy as you please.
The quickest and easiest way I found to do this mission is to make your way to each location via the streets, then when you are just outside of the restricted zones, climb to the roof. Put any Gunners that you encounter out of commission as you move towards the green objective marker, then drop down next to the barrels, shoot them to blow them up and then quickly return to the rooftops to escape.
Repeat the process for all three zones and once all three of the target barrels of powder have been blown up, the mission will be complete.