Just before the document element, you associated the document with the DTD:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html> ... </html>
You can also just inline the declarations:
<!DOCTYPE simple [ <!ELEMENT simple (#PCDATA)> ]> <simple>Super!</simple>
The example above is called the "internal subset".
The "internal subset" is where you put general entity declarations.