Insert the merge field: [[Signature]] into any template in order to include your personal signature in that template. To setup your signature find/create the template that has the title of: "Signature" and make changes as needed.
See the video below for step-by-step details:
This video shows how to setup a basic "Signature" Template that can be used in other email Templates that uses the Merge Field [[Signature]].
Be sure to edit the "Signature Enhanced" Template as well, since it will be used with the emails that were created using the Enhanced Editor. Also, be sure not to delete or rename either the "Signature" or "Signature Enhanced" Templates, or they may not work correctly in your account.
Take a look at these videos for more details on editing Templates!