Once you are ready to set up your email signature, open Postbox. From the top menu, click on Postbox and then click Preferences.
From the left menu, select Identity. On the right, select Postbox Signature and click the Edit button next to it.
Delete the default message that is in the text box and paste your email signature into it. Once you are done, click OK.
Your new email signature is now installed! Test it out by composing a new email to make sure it looks correct.