It's actually pretty simple.

In **view.html.php**, load modal library, if it's not already done:

    JHtml::_('bootstrap.modal');

In **default.php** or in any file in `tmpl` folder add:

    // Link for opening modal
    <a href="#modalDivID" data-toggle="modal" class="btn">Open modal</a>

and modal div, what will be shown after clicking on link

    <div class="modal hide fade" id="modalDivID">
      <div class="modal-header">
        <button type="button" role="presentation" class="close" data-dismiss="modal">x</button>
        <h3>Modal title</h3>
      </div>
      <div class="modal-body">
        Modal content here
      </div>
      <div class="modal-footer">
        <button class="btn" type="button" data-dismiss="modal">
          <?php echo JText::_('JCANCEL'); ?>
        </button>
      </div>
    </div>