1. Include sale.js in HTML page and configure it.

<script src="http://salejs.com/v1/cart.js"></script>
<script>
  cartjs.initialize({
    emailOrdersTo  : 'your-shop@mail.com',

    // Optional configuration options.
    language       : 'english', // 'russian' also available.
    currency       : '$',
    requireName    : true,      // Require name in contact form.
    requirePhone   : true,      // Require phone in contact form.
    requireEmail   : false,     // Require email in contact form.
    requireAddress : false,     // Require address in contact form.
  })
</script>

2. Add Cart Button

<a href="#" class="cart-button" style="display: none;">
  <span class="cart-button-quantity"></span>
  <span class="cart-button-label"></span>
  <!-- Or put an image here instead of span elements with text. -->
</a>

3. Add Buy Buttons for Products

<a href="#" class="cart-buy-button"
  data-name="Nexus"
  data-price="450"
  data-quantity="1"
>Buy $450</a>

or using JavaScript

cartjs.add({
  name     : "Nexus",
  price    : 450,
  quantity : 1
})

4. Optionally you can alter position of Cart Popup

<style>
  .bootstrap-widget .popover {margin-left: -10px !important;}
  .bootstrap-widget .popover .arrow {margin-left: 10px !important;}
</style>

Also, use this page as example