Go to https://developers.facebook.com/apps/ and press "Add new app"
Enter your forum name and email
Go to Products and add Facebook Login
Go to Settings > Basic.
Copy the App ID and App Secret.
Add domain names (make sure to add both the "www" and non-"www" version if applicable.
Add site URL. Use http or https, whatever you have standardized on. Use trailing slash, like this:
Go to Settings > Advanced. Add your domain to "Domain Manager" including http(s) and trailing slash.
App review -> "Make Public"
Go to your Forum admin panel. Go to extensions. Enable the Facebook extension. Enter App ID and App Secret as noted earlier.
In some cases, you have to run a
php flarum cache:clear and
rm assets/rev-manifest.json in the shell as a last step.
Your facebook login should work now!
(NB. will add more screenshots later)