The very beginning takes a bit of trial and error, just run for your life, and don't head toward any dead ends. Taking a very quick look around when you come to an intersection point between hallways can help as long as you don't take too long. Before long you'll get to a point where you're not being chased, and soon after that you'll be cut free of the straight jacket. From there all you have to do is keep moving forward through the level, regaining your abilities from the first game along the way. When you get to the first store, be sure you buy the engineer suit, because that's the only part of the standard loadout from the first game you have to buy in the second.