Login to Darwin's Dashboard and visit "Tracking Code" in the navigation.
Copy the tracking code HTML that you'll find there.
Login to Google Tag Manager and click on "Add a New Tag" button.
Click the "Tag Configuration" section
Select "Custom HTML" in the tag type list.
Add the tracking code into the text field.
Click "Triggering" section.
Select "All Pages" as trigger.
Name the tag and click "Save".
Make sure your live environment is selected, and click "Publish".
Once you've saved your changes, you can verify your install on the Darwin dashboard.