Set up your mailbox(es) using the Spark tutorial:

Once the application has been installed and set up, all you have to do to configure your signature is go to the "Settings" menu:

Select the "Signatures" tab:

In your user interface, under "Email signatures", select and copy the "HTML code" displayed below the signature:

In Spark, click on the "Add a signature" button, then on the "HTML" button and paste your code :

When you exit "HTML" mode, you can also add text to your signature .

And that's it, your signature is set up, and will be automatically updated when you make any future changes.

Now you can test your signature by sending yourself an email BUT BEWARE that the link to your signature is NEVER active when the email is sent, only when it is received.