I started by the beginning

Ok, it's not that easy, but well, in fact it is

(fuzzy logic ?)
I mean that, as when you are learning a new programming language, you will start by this "Hello World". Same for demos, you will start with one rotating cubes and then you will try to add cubes, to morph cubes, to change colours and to add lots of polished things and then, it will looks like a real demo similar to the one you loves.
Do you see my point ?
Just go and play around. Take one idea (or one effect) try to reproduce it (we are all starting by reproducing stuff).
Don't forget to read on internet, technical papers, blogs, tutorials related to demoscene or not

If I can give you a starting point, you said you made some OpenGL. Did you look at shaders ? Did you learn to use these ? Did you look at FBO ?
There are the base of 3D applications nowadays.