Copy HTML Purifier helper to:
./application/helpers/htmlpurifier_helper.php

Download the HTML Purifier standalone version and copy to:
./application/third_party/htmlpurifier-4.6.0-standalone/*

Make the following directory writable by your web server:
./application/third_party/htmlpurifier-4.6.0-standalone/standalone/HTMLPurifier/DefinitionCache/Serializer
Permission 775 (-rwxrwxr-x) is usually sufficient, depending on your web server configuration.

Create and customize HTML Purifier configurations by opening the helper (htmlpurifier_helper.php) and editing/adding case statements as needed. There's a default configuration named "comment" that should be sufficient for basic web comments with HTML allowed, but be sure to check it and make sure it meets your standards.
View the HTML Purifier Configuration Documentation for more information.

Use the following code to purify using the default configuration:
$this->load->helper('htmlpurifier');
$clean_html = html_purify($dirty_html);
Where $dirty_html is a string, or an array of strings.

To use a custom configuration, pass the name of the configuration in the second parameter:
$clean_html = html_purify($dirty_html, 'comment');