There are preformed systems purpose made for this type of consturction, use them, trust me they are easier to fit, have all the components required and make it easy to produce a water tight substrate.
My first choice would always be this system:-
http://www.wetroom-solutions.co.uk/If that doesn't float your boat, Wedi also produce a system called Fundo:-
http://www.wedi.co.uk/fundo.htmThe only time I'd use any plywood is for a reinforcement before either of these systems were used.
Tiles don't like things that expand and contract, ply expands and contracts and there are far better products available for the wet areas.
Do not be put off by the prices of these systems, by the time you've bought the materials to construct your own system, bought the drains, bought the tanking system and paid a guy to construct it from scratch it will be no cheaper, probably less accurate and probably more prone to failure.
Don't short cut the most important part of the installation.