Install modman - https://github.com/colinmollenhour/modman
Run the following modman command:
$ modman clone https://github.com/Metrilo/metrilo-magento1-integration -b 'APIv2'
Configure the plugin by going to "System" -> "Configuration" -> "Metrilo Analytics"
You can take the credentials from the Settings/Installation page of your Metrilo project.
Flush your magento cache.
The last step importing your historical orders by choosing the store on the "Current Configuration Scope", while still on the Configuration page -> Metrilo Analytics, and hit the Import button (if you use a Magento MultiStore installation, you will need to make the import for each store separately).