How to send emails using multiple domains with Amazon SES

I’m build a mini sendgrid alternative. I want to use Amazon SES to add multiple domains to my account so that my clients can send their emails from their domains. What are the steps (via API) to be able to add these multiple domains to my SES account so that I can enable my clients to send their emails? I’m guessing I will have to validate these domains first. Is there an API to do that? And then what else?