WordPress.org

For an automatic installation through WordPress:

1. Go to the 'Add New' plugins screen in your WordPress admin area
2. Search for 'Unfiltered HTML for Editors'
3. Click 'Install Now' and activate the plugin

Or use a nifty tool by WordPress lead developer Mark Jaquith:

1. Visit this link and follow the instructions.

For a manual installation via FTP:

1. Upload unfiltered-html-for-editors/unfiltered-html-for-editors.php to the /wp-content/plugins/ directory
2. Activate the plugin through the 'Plugins' screen in your WordPress admin area

To upload the plugin through WordPress, instead of FTP:

1. Upload the downloaded zip file on the 'Add New' plugins screen (see the 'Upload' tab) in your WordPress admin area and activate.