1) Download the application. This content will be a compressed file.
2) Extract the contents to your work area.
3) If you have renamed your admin folder on your site for security purposes, you will need to rename the admin folder within this application to match the new folder name.
4) Using either your favorite FTP client or the tools provided by your hosting service (if applicable), upload the files to your sites root folder making sure that you keep the folder structure intact.
5) In your ACP, you will need to goto Application & Modules -> Manage Applications & Modules.
On the top right you should see the application with an "install" button just to the right. Click the button and follow the prompts.
6) After the installation is complete, goto the applications settings and set it up to suit your needs.