How do I incorporate paypal into the application?

 Am busy with a shopping app here so am having issues with the checkout of items, like when the user needs to make a payment, so how do I incorporate PayPal to my application? 
 
Because what am actually trying to do is to perform a payment with 3 steps:

1) Order Page with PayNow Button for users to click

2) PayPal Payment

3) Redirect to "Payment completed page"

I have tried to implement this:

Paypal.Button.render({
  //Configure environment
  env: 'production', // To test, set to `sandbox`
  payment: function () {
    // Set up the payment here, when the buyer clicks on the button
  },
  onAuthorize: function (data, actions) {
    // Call your server to execute the payment
    if (error === 'INSTRUMENT_DECLINED') {
      actions.restart();
    }
  }
}, '#paypal-button');