By default FGLayout creates an external CSS file and adds a link to it inside your document's <head> tag. This is called a dependent file. Source code tab is your main document. The CSS tab is your dependent file.
It's important to work in a defined local site folder otherwise the path to your assets will not be correctly mapped to your site. Paths will point to assets on your local computer which nobody except you can see.
There's no shortcuts to creating web sites unless you use pre-built templates or features like FGLayouts. What do you learn from that? Not much. Shortcuts are fine AFTER you learn the fundamentals. Until then, you should do every step on your own. Expecially since you're earning a grade on this project.
Start with a basic HTML5 document.
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>XYZ Project Name</title>
</head>
<body>
INSERT CONTENT HERE....
</body>
</html>
Add HTML5 tags to your <body> to hold your content.
<header> </header>
<nav> </nav>
<article></article>
and so on.....
Then create a CSS file and save it to your site folder.
xyz_styelsheet.css
Add a link to it in your HTML <head> tag.
<link rel="stylesheet" href="xyz_styesheet.css"/>
Save your main document.
EDIT: You may copy text from your old project into the new one. But I would avoid bringing the other code into your new document because it would just result in more of the same.
Nancy