PREREQUISITES: Make sure you check HTML Purifier and make sure that you
have fulfilled all of its requirements before running this.  Specifically,
you'll need the PHP extension ctype (in almost all PHP distributions),
and it's nice to have dom and iconv.

* Place the htmlpurifier folder in your drupal modules directory.

* Download HTML Purifier from http://htmlpurifier.org/ You will need
  4.0.0 or later.

* There are two possible ways to install the HTML Purifier library. 

  1. Module directory installation. This means installing the library
     folder under the module directory. This way has the advantage of
     not depending on other modules. The issue is that when you
     upgrade the htmlpurifier module the HTML Purifier library gets
     removed and you have to re-extract the archive in the newly
     installed module directory.

  2. The preferred way is making use of the libraries API,
     http://drupal.org/project/libraries. This makes the library
     available to all sites or to a specific site in a multisite
     Drupal setup. You'll need to download the libraries API module
     and enable it before enabling the htmlpurifier module so that in
     the install phase it can find the library.

     Extract the library folder to sites/all/libraries or to
     sites/<site>/libraries for a specific site in a multisite Drupal
     setup.

     The final setup should be, when making the library and module
     available to all sites:

     sites/all/libraries/htmlpurifier/
         HTMLPurifier
         HTMLPurifier.autoload.php
         HTMLPurifier.auto.php
         HTMLPurifier.func.php
         HTMLPurifier.includes.php
         HTMLPurifier.kses.php
         HTMLPurifier.path.php
         HTMLPurifier.php
         HTMLPurifier.safe-includes.php

     Now you can safely upgrade your htmlpurifier module without
     having to re-deploy the HTML Purifier library.
         
* Go to Administer > Site building > Modules and enable this module

* You can now create a new input format or add the HTML Purifier to an 
  existing input format. It is recommended that you place HTML Purifier as 
  the last filter in the input format. Reorder the filters if necessary.

WARNING: Due to HTML Purifier's caching mechanism, dynamic filters MUST NOT
be placed before HTML Purifier.