Try http://www.pascospecialty.com/prod04.htm for a graphical illustration and http://www.taunton.com/ as a source for a book and video by Michael Byrne on setting tiles and, particularly shower pan installation.
I'm not qualified to help you much further. I've just been researching; getting ready to do my own first installation. I think you may be confusing the membrane to protect the wall (usually tar paper or a plastic sheet membrane behind the backer board but some membranes can be troweled on or applied over the board) with the shower pan liner (40 mil pvc or cpe) that is sandwiched between a a sloped mortar bed and a thicker sloped mortar bed which the floor tiles would be set on. In any case, it's got to be done right, so you'll want to research carefully. I've gotten wildly varying bids on having a shower pan redone-from 25$ sq. ft. up to nearly $50. Good luck. Dave