Steer your Blackspy craft and follow your team mates until you get out of the tunnel. You need to evade some enemy mines along the way before finally reaching the command sub. Wait until you get the signal to plant a charge on the sub's tail. Wait until it surfaces and board it.
Kill all enemies on the deck then enter the hatch. Kill the next two enemies by the door then grab the intel on one of the bunk beds.
Continue fighting your way until you reach the missile silos. Kill all enemies inside and find the last of the two intels available in this mission.
Fight your way in the second silo chamber until you reach the bridge entrance. Breach it open and kill all personnel inside.
Easy day, Overlord