First, download the CSS stylesheet (right click on the link and select 'Save Target As...' in Internet Explorer or 'Save Link As' in Firefox):
Add the following code into the <head> tag of your page:
Set up your menu using the following format:
<ul id="nav">
<li>Menu 1
<ul>
<li><a href="#">Item 1.a</a></li>
<li><a href="#">Item 1.b</a></li>
<li><a href="#">Item 1.c</a></li>
</ul>
</li>
<li>Menu 2
<ul>
<li><a href="#">Item 2.a</a></li>
<li><a href="#">Item 2.b</a></li>
<li><a href="#">Item 2.c</a></li>
<li><a href="#">Item 2.d</a></li>
</ul>
</li>
</ul>