Copy the extension to phpBB/ext/restlessrancor/uptimerobotshield
Go to "ACP" > "Customise" > "Extensions" and enable the "UptimeRobot Shield" extension.
An account at https://uptimerobot.com is required to make an API key.
After setting up your site monitor, go:
My Settings > (Scroll down) > API Settings > Monitor-Specific API keys to create your API.