I need help understanding layered materials


I’ve been trying to understand “Layered Materials” but I can’t seem to get it for the life of me. I’ve searched google but I only ever find tutorials on how to make landscape materials and blendable textures. I know there’s a way to blend material functions, and after reading several forums, I know I’m on the right track, but the “alpha masking” or whatever you call it is blocking me (at least I think it is).

Here’s what I have so far, my goal is to have a simple Tile MATERIAL blend with a simple Moss MATERIAL. I used material functions for the two layers. I know I’m supposed to have something in the alpha slot but everything I try fails. The end material should be able to have the moss “painted” on by using the alpha painting tool.

Here’s the main Material:

and the tile Material Function:

and the moss Material Function:

Thank you for any help. I appreciate it.