Download Sur.ly to your desktop, extract the plugin folder from zip and upload it via FTP to the root directory of your FluxBB.
Run the install_mod.php file from the root directory in your browser. Click Install button, and once the installation process is over, delete install_mod.php.
Add the custom code into FluxBB files. NB: You may find another copy of the code within the Sur.ly's zip archive.
After the code changes are propagated, go to Administration → Plugins menu → Surly.