I plan to refinish our kitchen floor with ceramic. The current floor is hardwood over 3/4" diagonal planks subfloor resting on 2x8 joists 16" oc. The surface to be tiled is roughly 9’ x 7’.

My plan was to remove the hardwood floor and lay 3/4" plywood over the existing subfloor and screw everything together every 4”. I cannot really go any thicker than that because the floor would be too high compared to the adjacent dining room.
I have a few questions:
1. Is this the right way to go or should I use something else than plywood for the second layer? Or remove and replace the subfloor with plywood as well?
2. Should I use tong & groove plywood or leave expansion gaps between the sheets?

A better solution would be to use 1/2" layer of plywood (no tongue & groove), then use a tile underlayment like 1/4" Hardibacker, Durock, Permabase, etc., then tile. Alternately, you could step up to 5/8" ply, then use Ditra. Either option would add up to about 3/4".

I would avoid tiling directly to plywood, as it can be problematic at best. While it can be done, it has to be done strictly to specs. A tile underlayment is a wiser choice.

Thanks for the advice.