Usually, with a text file. Name it something like "website.css" and then start typing css codes in there. When you have enough code, you can call it a layout. Good looking is something else, in the eye of the beholder.
what do you mean by how do you get start? it is a simple text file with its special language, and it should be written step by step with your html to test if it is working or not for a single step. if you design all of html and then link whole of CSS file, it will be hard to find errors! (this is my method! )