I'm familiar with JS and AJAX itself (PHP, I guess, is still my trouble), but, looks like there is something more in Prestashop, that I should know about that to do so.
First, I am sending $.post to order-opc.php. Then, I am getting response like this:
url: '{$base_dir}modules/order-opc.php',
type: 'get',
data: 'ajax=true',
success: function(data) {
//Append order-steps to current page body
$('body').append(data);
But, as I see, this approach isn't working here.