Set up a ‘Google Tag Manager’ account: https://tagmanager.google.com.
Click on the ‘Tags’ tab and click ‘New’
Select ‘Custom HTML’ tag and enter a name for your tag.
Create a Facebook business manager account. https://business.facebook.com/
Go to the Facebook Events Manager and select ‘Add Event’: https://business.facebook.com/events_manager/
Select ‘Install Code Manually’ and copy the code snippet.
Past the code snippet into the HTML container you have created in Tag Manager.
Select the trigger to be ‘All Pages’ and click save.
Your tag should now show up in the list.