Adding an alias on your Gmail allows you to write emails from your alias but also to benefit from its Signitic signature.
Go to Gmail to start, in the general settings :
Then, in the Accounts tab, click on Add another email address :
Then you can enter your alias in the email field and leave the box marked Treat as an alias.
And that's it, you can now write from your alias and the signature of your alias will be available tomorrow.
In a new message, you can change the sender's email (1) or choose another signature (2) :