Let's assume you want to use some fonts from Google Fonts. You have chosen them and have the code you need for them to appear on your site.
In this site we have chosen some fonts and have the code:
Simple go to the header area on the menu
and paste it in to the box there.
You can go to the CSS area and let the system know your class details:
You can then add the code by pasting it in:
and it will appear on your pages menu in the admin area