I want to set up an email address for my domain. How do I proceed correctly?

Buy a mail package (e.g. "Mail Easy") - and then?
Please keep in mind that Mail Easy only includes forwarding. So you can't reply from the email address.
First you have to link the domain to the hosting package:
you can create any accounts to all domains in your domain list.
Once you have booked the package and the domain is connected, you can create the email addresses in Froxlor. That is described here (but for redirects). You can also just create real accounts, then you only need to set a password: