You will know you can approach this mission after a new distress flare appears on the Arkham City map #1, in the Amusement Mile district in the east. As the whole area surrounding it is flooded #2, you will have to make good use of the Grapnel Gun and gliding. You won't die upon falling into the water, but will lose a few second waiting to reach a safe spot.
You need to reach the Krank Co. Toys building. after reaching it you should note that the entrance is guarded by a group of inmates #1. Attack them by surprise and afterwards finish off in direct combat. Afterwards enter the building #2.
Follow the only possible corridor and soon you should reach a slightly bigger room #1. Here Batman will meet an old acquaintance - Bane - with whom he has already fought #2. This time you won't have to fight him however, instead having to search and destroy TITAN containers. You need to take care of six of them.
Destroying the first container
Note that the locations of the containers have been added to your map, though you can't yet access all of them (assuming you have taken on this mission at the first possible occasion). Two of them can be found in common areas and I'd suggest taking care of them first. The first container is in the south-west part of Arkham City, at the Subway Terminal in the Bowery #1. After getting there, jump down to the lowest level and search for the container #2.
It will be probably guarded by enemies equipped with firearms, so I'd suggest staying very cautious throughout the fight #1. Try to attack the enemies by surprise if possible and keep knocking guns out of their hands. After the fight equip the Explosive Gel (left on the d-pad) #2.
Approach the container and press the right trigger to spray Explosive Gel onto it #1. Afterwards detonate the charge by pressing RB #2.
Destroying the second container
The second container can be found in the south-east part of Arkham City, in the Industrial District #1. After reaching the spot, head south and look for a fenced area guarded by a few inmates #2.
It of course would be best to attack them by surprise #1. Luckily, unlike the previous group, they shouldn't be equipped with any firearms. Once you're done with them, spray the Explosive Gel onto the container and detonate it #2.
Destroying the third container
The third container is inside the Sionis Steel Mill building in the Industrial District of Arkham City. Head there and examine the southern wall of the building, finding a door leading inside #1 #2. Notice! If you're visiting this place further in the game, there might be some enemies in the area. In such case, check out the Infiltrate the Steel Mill (further part) walkthrough.
If you have chosen the proper passage, you should begin the exploration of the Steel Mill inside the Cooling Tunnel B. The container can be found nearby the starting point #1 #2 and shouldn't be guarded by anyone. Traditionally, spray the Explosive Gel onto it and detonate.
Destroying the fourth container
The fourth container can be found in the tunnel connecting the Subway Terminals #1. In order to reach it, I'd suggest heading to the entrance located in the Bowery, east of the Museum #2.
Depending on your previous actions, the subway station may be guarded by a numerous group of armed bandits #1, or only by a few. Regardless of the turn of events, you have to find the terminal entrance #2 and go down the stairs.
After reaching the subway terminal turn left and use the stairs #1. Soon you should reach the station, where you have to turn left (east) #2.
Use the Grapnel Gun to reach the upper platform #1 and continue going east, choosing the destroyed passage of your way #2. Eventually you will have to perform a slide.
Stop after reaching the barricaded passage. Turn on the Detective Mode and you should note that one of the walls is destructible #1. Choose the Explosive Gel, spray in onto the wall and detonate #2. Luckily you won't alarm the nearby enemies.
Now head east, jump on your way there and you should reach enemies guarding the container #1. You can attack them from above or quietly jump down and take down two of them at once. Afterwards choose the Explosive Gel, spray it onto the container and detonate #2.