For instance, I have some that will replace the vanilla ones around the UL Dark Forest. Where do I put them?

\Bethesda Softworks\Oblivion\Data\DistantLOD

Also, don't forget to run TES4LODGEN.

True. Better to run that, then use prepackaged LOD files.

That depends on what kind of LOD files they are. The ones for landscape go in Data\Meshes\Landscape\LOD. If you have the other kind, those go in Data\DistantLOD. The second type is better served by using Tes4Lodgen though.

The last two updates for Dark Forest don't include either one though, so it sounds like tes4lodgen is your answer there.