The process to create an email alias varies depending on the email service provider you use. Here are some general steps:
For Gmail: Go to settings, find the "Accounts and Import" tab, and click on "Add another email address". For Outlook: Go to "Settings", click on "View all Outlook settings", then "Email", and finally "Sync email". You can add an alias from there. For custom domains using G Suite: Use the admin console to add and manage aliases.