You don't have to take any of the door apart really. The rubber around the window frame just pulls out and then you can fold it around the white plastic thingy and push it all back in together.
I'm not sure exactly how the plastic bit locates in the window as I broke a small plastic "pin" off mine when I did it. There's a little hole in the top corner that probably has something to do with it which I put a tiny screw through to hold it all together.