To install Code Extinguisher, simply unzip the contents of the downlaod to your CodeIgniter directory (Alongside index.php). Your directory structure should now look like this:
codex/ (the codex files)
system/ (the CI files)
backend.php (the codex redirector)
index.php (the CI redirector)
installer.php (the codex installer)
Run installer.php from your browser and follow the on-screen instructions.
Note: You will need to have write permissions on the database.php file (located in codex/application/config/) as well as the logs/ directory.
The installer simply updates your database.php file, and sets up the database for you. If you wish to run the installer manually, then execute the sql statements in the codex.sql file (which is included in the zip), and then update your database.php file.
Later, when you want to access the backend, simply direct your browser to http://yoursite.com/backend.php
note: if you have enabled friendly URLs via .htaccess, you will have to add backend.php to your exceptions, like so:
RewriteCond $1 !^(index.php|backend.php|images|assets|robots.txt)