Once declared, you can use it by referencing the DTD in the DOCTYPE:
<!DOCTYPE html PUBLIC "-//MyOrg/Widgets 1.0//EN"
"http://www.myorg.com/DTD/widgets1_0.dtd">
<html>
<head>
<title>XHTML with Widgets!</title>
</head>
<body>
<p>This is an example document using the
cool new widgets from MyOrg.com:</p>
<widget type="spreadsheet"> ... </widget>
</body>
</html>