How do I set up a mac.com email account?

All dot-Mac email accounts were mapped to the iCloud.com domain. If in years past you had been using john@mac.com for example, add it to Mail by following these instructions:

Add and remove email accounts on iPhone - Apple Support

The email service to choose is iCloud. You will receive email messages addressed to john@mac.com, john@iCloud.com or john@me.com. They are all aliases of one another.