Start Winter Contingency at Mission Start, and continue through the level until reaching the 'main' building (at the first covenant encounter). Exit the building, and immediately turn left. Walk around up to the left side (right, when facing the building), near the two cylindrical structures. Jump on top of the ledge on the front of the building, and walk on it towards the center. You should see a line of Lights hanging from the top of the building. Jump onto one of these, then on top of the building. (If you are having trouble reaching the lights, continue along the ledge until you reach a small block, that is slightly higher. You can make the jump easier from here). Once on top of the building, walk towards the back of it (where you originally came from when starting the level). You should hit a barrier. Back up, and jump on top of this barrier. You should now be floating above the building.
You can move anywhere on this barrier, as long as you stay above a structure. If you move towards the center of the courtyard area, you will fall to the ground.