You start by reading the following sections

Admin > Manage Styles
Click Create Style
Type a name for the new style in the box - scroll to the bottom and save
Click Modify Style
Choose the style you created - click Pick Style
Scroll to the bottom - erase all of the existing css in the box - paste the new code in.
Click Modify Style
To use your new style choose it in the dropdown menu from Manage Styles.
   

Stylesheet Making Tips/Helpful Links
   

Stylesheet Making Tips/Helpful Links

Take your time and do it step by step.