I just installed 3/4 inch plywood over concrete and will be installing 1/4" hardwood on top. The plywood isn't perfectly flat, there are high spots, some of the boards are 1/16 to 1/8" thinner than others, and the boards slope down about 1/4" per 5 feet at the walls. Some of this is from the slab, some is from the plywood. What's the best way to level this before i put down the hardwood. I will be using #15 tar paper rolled out over the plywood before installing the hardwood.
I've heard using shingles in low spot and grinding the high spots works. How perfectly flat does the plywood need to be before laying the hardwood?

Did you do a moisture test on the slab first? Vapor retarder under the plywood? Floor prep to industry standards first, then vapor retarder/s ( i use two) then plywood.

All your leveling should have been done to the concrete, before the plastic vapor barrier and plywood.