The twelfth mission of Metal Gear Solid V first needs to be unlocked, because it does not become available automatically from the map of the main missions. Again, take interest in the same Side-Op, as in the case of your preparations for the eleventh mission, i.e. Make Contact with Emmerich. As your starting point, pick, e.g. the landing zone in Aabe Shifap Ruins (this is where you fought Quiet) and head Eastwards, while passing by several enemy outposts. Your destination point is Serak Power Plant.
You need to reach the red door in the Eastern part of the power plant and it is best to do this without raising the alarm. I recommend, therefore, that you move glued to the rock on your left (additionally, the best time of the day is the night) and either walk around, or knock down only those of the enemies that visibly stand in your path. Along your path, climb over several fences, which should be no problem to you. Interacting with the door in the power plant ends side mission 82 and starts the twelfth main mission (Hellbound) automatically.