Hi sayre_icloud,
Thanks for using Apple Support Communities. You can create custom email addresses for domains you already own after you have set up the custom email domain for iCloud Mail. So, start with the guidance in Add an email domain you already own to iCloud Mail on iCloud.com including these basic requirements below.
"Before you begin
Once you have set up the domain for iCloud Mail using that article, you can move on to Create and delete email addresses for a custom domain on iCloud.com in order to create the email address. Guidance is as follows.
"Create an email address
You can have a total of three active email addresses per domain.
- On iCloud.com, click
in the toolbar, then click Custom Email Domain. - Select the domain you want to create an address for, then click
next to your name. - Enter the new address in the text field, then click “Add email address.”
Note: You can’t add an email address if you or someone else is using it with a different Apple ID. If you want to add that address, the other Apple ID needs to be changed to a different email address. See the Apple Support article Change your Apple ID."
Take care.