MikeJones
Wherever you registered your domain name, you point that to your AWS instance.
I would practice installing and destroying a instance or two while you get the hang of using Flarum and getting accustomed to AWS and domain registration before you set it up as a production server .
Create instance
You will need to enable http and https as I believe ssh is already enabled by default.
Run the script , I believe you need to use the sudo command.
Name the server the IP of your instance
You can always change it later. I'm not sure if your using a dynamic or static instance , most likely when you reboot your server it won't come back with the same IP .
Review :
https://docs.aws.amazon.com/Route53/latest/DeveloperGuide/routing-to-ec2-instance.html