2. Upgrade your account to Premium while using a browser (button in the top menu).
3. Add your domain in "Settings" -> "Extensions". There you will also find instructions on how to configure your MX DNS entries first in order to route incoming SMTP emails to the Tutanota email servers.
4. Add user accounts (i.e. mailboxes) for your domain in "Settings" -> "User management". If you have a large number of users, you can import a CSV file containing user name, email address and optionally the user password. This is useful to automatically create all user accounts in one step.
5. Provide the user passwords to the users, so they can log in into their mailboxes.
Additionally, you can find a manual for Tutanota here: https://tutanota.com/howto