Go to the "Extras" tab of the mailbox; click on "Tie to Identity," click in the "Identity" pull-down and pick "New Identity."
Give it a name; pick the SMTP server you want it to use; the "Inherit from" pull-down has several choices--if this is your main account, choose "Default Identity" and any accounts you don't specify an identity for will use these settings. Fill in the "From" tab: click on "Active" and put in your name and email address. Since these are the fields that will show up in your mail header, whether you want to fill out any more of them is up to you. Except…
…if you want to use a Signature, select the "Options" tab. Make the "Signature" box active, click the "Signature" button and type in the signature you want to append to every email.
Good luck with the rest!