Where do declarations go?

In a <STYLE> element in the HTML file's <HEAD>:

<STYLE TYPE="text/css">
P { marginRight: 0.5in  }
</STYLE>

Declarations can also be placed within a <STYLE> element. If the <STYLE> element is in the <HEAD>, the entire document will be affected; if the <STYLE> element is in the <BODY>, only the subsequent HTML elements will be affected.

It is appropriate to place a style sheet inside an HTML document when you want to email the style sheet and HTML data together as a single file, or when you design a unique style for a document which you do not plan to reuse elsewhere.