Steps

  • In Email Signature Designer : Install > Copy to clipboard ;
  • Open Apple Mail app ;
  • Open Preferences… panel ;
  • Select the tab : Signatures ;
  • Select the account you want to install the signature ;
  • + to add signature ;
  • Unselect Always match my default message font ;
  • Paste you signature from the clipboard.

You can now compose a message with your new signature.