How to import custom tiles?

[ZugZug2151]

So apparently reforged has changed the file types or something. I tried importing .blp tiles to replace Lordaeron Summer grass but it doesn't work. Then I read that there are three new files for textures (diffuse, normal and orm). So can someone explain what those are and what do I need to know to successfully import a custom tile? Do I need to have three separate imports? Can I do it the old way and just ignore the extra layers and import the main tile? What am I missing?