First, go to the helicopter and, while still on the platform, go to the middle of the map. Look up and you find a small wooden platform.

Jump on it and follow the platforms until you get to the gas tank. Look out of the map and you will find another platform. Jump on it, run towards the crane and double jump on it.
Next, go to the right and follow the path to the other platform.

Then pass it and crouch and follow the rail to a small window and there.