In the pay_form comments of pay.module, it says:

* A simple form any type of pay element.
 * If you want to build a module with a standalone payment form, you would
 * do so by passing this function to the drupal_get_form page handler in your
 * hook_menu() funcion.
 *
 * Alternatively, you can manually add payment form capabilities by including
 * the code from this function in your form builder or form_alter code.

When I try adding the page callback as pay_form, or, including the code in my own form, I get:

Fatal error: Call to a member function form() on a non-object

Which is referring to $pay not being an object.

How can I simply attach payment fields to a custom form using Pay?