Add more email accounts

Settings, Mail, Accounts, Add Account

Enter the type (Google, outlook, whatever) then your new email address, then the password when asked.

Some accounts may want you to create a temporary app related password, some require a two-factor authentication cofe or app. Google your provider for info.

Are you talking about an AppleID for her phone? Because yes she should have her own ID on her phone.

She can either create it during phone setup or if the phone is already set up, create it in appleid.apple.com (on the Mac) and sign out of the old ID and into the new ID on the phone settings.

When you add her new email to your mac's mail app it will send a 2FA request to her phone and she will OK it and you type in the code shown on her phone.

I have an email account on this Mac and my iPhone.

I want to create an email address for my wife to use on this Mac and her iPhone