Guidelines to create textures for materials in Unity3d

I'm facing a problem with realizing particle systems.
Following many tutorials including the recent live session everything end up assigning materials to particle to get custom shape ones.

What I'm having troubles to find is some guide to create those materials or, for the sake of being clear, to create textures to assign to the materials in order to have the desired shape.

Browsing the internet I thought I found something useful until I noticed that the link in the answer is reserved to authorized reader, making it a totally useless answer (no quotes of the contents at all).

By downloading some free cool asset like the Elemantals, I learned a lot of things on how to combine different systems in order to get complex and spectacular visual effect but, as previously said, everything ends up with the need to create my own textured materials if I want to obtain something original.

So my question is:
where can be found guidelines in creating textures for particle materials? Is there any keyword I should look into (*like texture mapping, uv mapping, bump mapping, normal mapping* and so on) or (even better) any good tutorial straight to the point?

 

Has anyone ever dealt with this?

 

Can you provide an image or specify what you are attempting to achieve?