I know this is a completely different car but i work at the BMW Mini factory, and the hardest thing about changing a headliner is getting it in the car, when the car is made the the headliner is put in through the front windscreen which means you don't have to bend it to get it in, but when replacing it you have to put them in through the boot which is smaller so they have to bend and crease because like previously said there made of foam and cardboard basically and sprayed with stuff in between the layers to make it stronger and hold its shape. When fitted, fitting the trim back up should be easy providing its all lined up correctly. It might be easier on Leons as the boot opening is bigger then the Mini, just thought i'd say from my experience with the Mini.