Plan on 1/4 inch per foot slope. The slope is built into the bottom layer of the base, underneath the liner, then the top layer is a consistent thickness.

A great way to do this is to purchase a waterproof ready for tile shower base (it's much easier and better quality than trying to do a mud base). This base can be made any size and the drain can be put exactly where you want it (see there is no need to have to move the plumbing). I've included a picture here but I'll also include a web link to show more about this product http://innovatebuildingsolutions.com/products/glass-block/glass-block-shower - this type of base can be used for any type of wall - clear glass, glass blocks etc.