It is easy to go to a big box store and purchase a preformed fiberglass shower base. While functional, these bases come only in predetermined sizes. In addition, they lack the style of a tiled base, which yields a custom home look.
If you decide to build your own base you need to take into account several factors. The walls of the base must be constructed from a water resistant material such as Durock, which is a cement based board. It is important to use a rubber shower liner as a moisture barrier for your base. This liner must be properly installed and clamped onto the drain to seal off that vulnerable leakage area. When finishing the concrete it is important to gently slope it towards the drain. The slope has to be great enough to facilitate water flow, but shallow enough so that the person who is taking the shower is unaware that the floor is tilting.
It has been my experience that using a waterproof mortar does little to prevent seepage,, and so I usually use a standard mortar. You can expect the concrete below the floor tiles to get wet, which is why it is so important to properly install the rubber liner.
When done properly a custom shower base allows you to create a shower of any shape or size. In addition, it will give you a custom look that you cannot achieve using a preformed fiberglass base. A little care and planning will give you a finished shower that will last you decades. See photos below.