Navigate to Settings > Templates > New Template > Email Signature Template:
Add a name for your signature under Template Name, and copy and paste your signature to the field marked Template:
Tick "My default signature" to ensure this signature is added to all emails you send. You won't see the signature while you are drafting an email, but it will be visible once your email has been added to the Queue.
Some email types such as ad hoc Tax Payment Reminders can be sent by any staff member, but if you are the client's manager then this email will be sent from you. Tick "Available to all users" to ensure your signature is added to emails that are being sent on your behalf.