I updated this blog on 6/20/2021 to reflect the Summer 2021 enhancement. Salesforce now provides a default CDN domain for your salesforce instance. (Nice!)
Because Salesforce made setup easier, you can get complete Email Builder setup instructions in Account Engagement Email Builder – Helpful Set Up Guide
Good luck!
More
Other helpful Email Builder blogs:
Account Engagement Email Content – Sending an Email
Account Engagement Email Builder – Explore Your Image Hosting Choices