Every DNS provider will be unique to some extent, but the principles that guide this process remain the same. As a result, most of the terminology remains the same as well. This guide may not lead you through every single step with your DNS provider, that is something you will need to know how to do. However, it will show you how this was accomplished for our sample scenario.
You will need to create a DNS record using the tools provided to you by your DNS provider, usually this is whomever you registered your domain with. You will need to consult the documentation for your specific domain registrar.
You need to repeat this process for every domain, as each domain has its own DKIM key.