STEP 1: Click on File and then Options in the bottom left (see screenshot below).
STEP 2: Select Mail, click on Signatures (see screenshot below).
STEP 3: Below the Select Signature to Edit box, click New (see screenshot below). Enter a name for your new signature. If you want to edit an existing signature, just highlight the name of the signature you would like to edit and it will appear in the Edit Signature box below.