How to build a CSS template from scratch

I want to build a CSS template of my imagination: this means from SCRATCH.

I want to do it like if I'm doing it with a pen, but just with the mouse. I have no idea about this field (css),that's why I'm asking. Many developers had made fantastic templates that rocks.

I want to know how do you create your CSS templates? (if you did) Do you start them from scratch, using a framework, using an application? Is that hard or easy?

Simply how you do it?