How do I install a really simple SSL for a Wordpress Site on AWS?

Check this

https://lightsail.aws.amazon.com/ls/docs/en_us/articles/amazon-lightsail-enabling-https-on-wordpress

Hello, In what type of compute resource you are installing your application? If it's on Lightsail the link provided will guide you through. If not, the most simple way is using Certbot, here you can get all the instructions. Please note, that this is an external tool, and will be installing the certificate on your server, in the shared security responsibility model.

AWS provides a service AWS Certificate Manager that creates and manages certificates and has integrations with load balancers and CloudFront, per example.