Open Gmail and sign in
Click the gear icon
Select See all settings
Open the Accounts and Import tab
In the Send mail as section, click Add another email address
Enter your name and the alias email address
Click Next Step
Enter the SMTP server details if required
Click Add Account
Verify the alias email address using the confirmation email
Return to Accounts and Import
Set the alias as the default send-from address if needed
Use the From field in a new email to choose the alias