From your dashboard, on the left-hand side click "Communications" and then "Dashboard" from the drop-down list.
Select "Signatures" on the left-hand side.
This page will be your list of Signatures that you have created. To create a new signature, click "+ New Signature."
From here, you will be prompted to enter all of your contact information for this signature as well as even add a real signature if you please. These signatures are possible to be made global for all admins to use. Click "Save" and this signature will be added to your list.