Trying to install large pieces of Kerdi should be left to experienced people...it is just too hard for the average DIY'er to key in, notch and then cover large areas with a single sheet of Kerdi before the thinset skins over. The videos on the Schluter website talk about this. Not pulling the membrane back to verify proper coverage is the main reason why a seam will leak - the thinset is either too dry initially, or it skinned over before it is covered and this prevents proper bonding. The same thing will happen with tile if you spread too much thinset out...the tile's won't bond well.