How may you make your own terrain type?

[GodOhNoPls]

I would like to make a gravel terrain plugin since there aren't any but I don't know how to even start so therefore I am asking for a nice way to make a terrain type!

Also would like to make it good at the coast like the in game ones instead of just the texture...
these are not mine i don't own these plugins

This one, according to me atleast. Is quite nice. And, Unique.

The one, I don't want. Ugly, and is just a picture or image/decal. Nothing special.

Normal terrain, with sand obviously probably what I would choose

so credits to whoever made those I don't remember THEM (it's multiple people)[/lb]