I don't do one or the other. I start with one element at a time.

What's your first element? a container that holds everything?
k...set up your html document with the div, and give it an ID name.

open your CSS document and write the code for that div.

Got a header? put a div inside that container you just created and give it an id of header.

go into your css document and modify the height and any other elements you can think of.

As you go along, you will modify the CSS for text (size, color, etc)... There's no "right way"...only your way. experiment around and do what works and you're comfy with