Creating a new materials dgnlib

I would like to have some extra materials to choose from other than the ones provided in the Workspace, so I'd like to create a new Custom_Materials.dgnlib.

I have a couple of png-images for a brighter asphalt, grass and gravel. 

What is the best workflow to create it?

Where must the new dgnlib be put? In the Standards>Materials-folder? What environment variable points to it?

Where do the png-files have to be placed? In the Standards>Material>bump and/or in the Standards>Materials>pattern folder? Or are they saved in the dgnlib?