There are a bunch of steps in preparing a shower for tiling. None of them really hard, but miss one, and you could be in for some problems. That being said, the first big mistake people make is putting the liner flat on the floor. The whole idea of the liner is to catch errant moisture and direct it to the drain (there are weep holes under the main grill in the body of the drain to let them drain out). This doesn't happen if the liner is flat on the floor until there is a significant accumulation of stagnent, smelly water trapped there.
So, I highly recommend you check out www.johnbridge.com and review the instruction in their "liberry" and ask questions on the board for those things you don't understand.
Basically, a shower pan with a liner is made up of 5 layers - a preslope, a liner, a final setting bed, thinset and tile.