To use a custom sidebar file, create a file named user-sidebar.php and drop it into the top-level Tarski directory, usually something like this:
yoursiteroot/wp-content/themes/tarski/
The custom sidebar file will be automatically picked up and used by Tarski.
The main advantage of using a custom sidebar file is that you can write your own code from scratch, giving you the power of a dynamic sidebar without having to change the core Tarski code (which otherwise might make it hard to upgrade).
Remember to back up your user-sidebar.php file whenever you update WordPress or Tarski!