Place your code on the page and surround it with <pre> tag. Set name attribute to code and class attribute to one of the language aliases you wish to use.
<pre name="code" class="c-sharp"> ... some code here ... </pre>
NOTE: One important thing to watch out for is opening triangular bracket <. It must be replaced with an HTML equivalent of < in all cases. Failure to do won't break the page, but might break the source code displayed.
<textarea name="code" class="c#" cols="60" rows="10"> ... some code here ... </textarea>
There's a way to pass a few configuration options to the code block. It's done via colon separated arguments.
<pre name="code" class="html:collapse"> ... some code here ... </pre>
For optimal result, place this code at the very end of your page. Check HighlightAll for more details about the function.