I know you said you're following that link and method, but we know you're not doing that yet.
The shower membrane used in a traditional method is 40 mils, not 15mil poly and is sitting on a pre-slope, with another 1 1/2" or thicker deck mud over it which will be the base for the tiles. Or you could go with surface membrane like Kerdi. The way you described your intentions it seems you'd like to do a method similar to the Kerdi, but you have the wrong products.
Go back and study, then come back. For what it's worth I recommend doing a Kerdi shower with Kerdi drain etc.