There’s a pic above from Arkiras on the material. This goes on your landscape. Height is derived as normal per what you might use for tessellation but is fed into WPO instead so the landscape can do collision for you.
WorldPosition carries over that math into the RVT output node to be sampled by other-stuff.
Be sure your landscape is set to render to at least the height-RVT you set up and then sample from that same on your VHFM (details panels in the editor). Be sure to copy-bounds on all the RVTs so all the shtuff lines up properly.