i think the regulator has to be off the glass in order to put it in. he probably cut the inside of the door.
once the regulator is off, you put the front edge down inside the door from the outside, then work it down until you can get the back bottom corner in. then you should be able to start rotating the glass back up to a normal position. once the glass is in, then you slide the regulator in from the bottom of the door, under the door panel.