WordPress.org

1. Download and extract the ZIP file.
2. Upload restrict-login-by-ip to the /wp-content/plugins/ directory
3. Activate the plugin through the 'Plugins' menu in WordPress
4. Go to Users -> Allowed Login IPs configure the plugin.

Note : If you accidentally lock yourself out of your site, edit your .htaccess file to remove everything that refers to "RestrictByIP". Also edit or delete the .htaccess file the plugin has created in wp-admin.