First off, the way the SEUT MatLibs are generally set up is with spheres that display each material contained within the MatLib. This makes it easier for users to find and select the material they want. But this can be any type of mesh really. Either way, create an object to assign the material to.

Create your material by referencing the  Material Creation Tutorial or by using Blenders File --> Append-functionality to import the material into the file.

Open

Assign your material to the mesh used to display materials in your MatLib.

Repeat this process for as many materials as you would like this MatLib to contain.

Right click your materials on the material list of the object each is assigned to and use Mark as Asset. This will add your material to the Asset Browser.