Nancy, great that you will be doing your tile! I did both my bathroom tile jobs. I used ditra. My floor was different than what you are planning. I had old diagonal subflooring, topped with fir tongue and grove. That was my floor, after I removed the asbestos ladened floor covering.

I was going to install heated flooring, and natural stone. I needed to check for the deflection of the joist to see if they were strong enough for stone. It takes stronger joist system for stone than for ceramic.

Next I mounted 3/8" exterior grade plywood with screws 4" on center throughout the field. I think the screws actually only need to be 6" on center though. Your goal is to get a solid floor mounted to the subfloor, not into the joists. There are also thoughts on where to place the seams over the existing floor. The seams should not line up IIRC. My warming wires were added next, then everything primed as per instructions, and self leveling compound poured over the floor. Now I had about another 1/2" worth of thickness.

Ditra (uncoupling membrane) went in next. I did not water proof the seams. They now have products that are thinner and can also accommodate warming wires, so you achieve a thinner profile. When I was using the Ditra, it was required that you use an unmodified thinset, I believe. What ever system you use be sure to follow the recommended products. I used top of the line tile installation products on everything. There are other products now that people have really liked.

Next I laid my tile using modified thinset. Grouted with epoxy grout. I never regret using epoxy grout. It has been so durable and stain resistant. I have a hard working bathroom on our farm. DH is in there with dirty shoes and grime. The floor grout is as nice now as it was when I put it in 5 years ago.

I may have missed some steps, but I think I got the basics. I hope others contribute. There are several professionals on the forum that give excellent advise.

It is common to screw 5/8" exterior plywood to the subfloor, at 6" intervals. Then apply thinset, and 1/4" cement board - screw that down also. You are now ready to tile. This will ensure the floor won't flex, and your tiles won't pop off, or the grout lines crack.

If the floor is too high, check out Schluter - they make tile edging and transition profiles (that's what you want).

As has been mentioned, for a bathroom floor, it does not have to be waterproof.

Nor do you have to use a membrane or mat like Ditra or other equivalent materials.

If you're using a small format mosaic, because of its waffle structure, Ditra prohibits using tile smaller than 2" on top of it.

In general, due to bonding and differential movement issues, you don't tile directly on top of a wood subfloor. It's common to have a transition material between the wood and the tile. That material can be 1/2" cement board as mentioned by geoffrey, or Ditra (or equivalent) as used by Enduring.

With proper subfloor in place, a low cost and fairly straightforward method is to use 1/2" cement board over your subfloor as mentioned by geoffrey. You then tile right on the cement board.

Sometimes the half-inch thickness of the cement board can cause elevation issues with the tile meeting other flooring, cabinetry, plumbing fixtures, etc.

That tends to lead people towards membranes like Ditra, or a membrane from Nobel, or if the floor is sloped and you want to level it, using a self-leveling compound.

Membranes like Ditra and Nobel are fairly thin, only about 1/8" in thickness. That can help minimize elevation gain. The membranes can also act as uncoupling membranes, they help isolate subfloor movement under the membrane from the tile above the membrane. That can help minimize cracking, etc.

If you're going to DIY, no worries. You just want to have a clear idea of the steps needed before you proceed. If you're going to sub it out, no worries. You still want to be educated in the steps involved, especially with the waterproofing in the shower. I'd venture that a tile sub hired through the tile supplier would have a decent track record, hopefully the tile store vets their subs prior to recommending them to customers. But I've been on these forums long enough and seen many cases of tiling stores turning their backs on customers when installations have gone awry.

Biggest error I see on these forums when installing mosaic is a very slight misaligning of the sheets. When grouted, the individual 12" squares of mosaic are easily visible to the eye. I never accept deformed boxes of mosaic tile from any supplier. If the box itself is askew, often times the "squares" of mosaic inside the box will be deformed slightly out of square.

Good luck with your installation.

I might only add that you only need 1/4" Durock on the floor. 1/2" is reserved for walls...

You Go Girl!