1. Creating REST APP to get Client ID and Secret Key from your PayPal sandbox account:

Log in to https://developer.paypal.com and click Dashboard tab.

Click My Apps & Credentials -> On the My  Apps & Credentials page -> REST API apps, click     Create App.

On the Create New App page, provide an app name and select Sandbox business account created in Step 1

Click Create App, and then note down Client ID and Secret Key details

Visit for more: https://developer.paypal.com/docs/checkout/integrate/prerequisites/#get-rest-api-sandbox-credentials

You will get Sandbox credentials by default use the following switch show in the below image to get Sandbox or Live Client ID, Secret key details.

2.  Add Smart Payment Button using following link


You can check Interactive Demo here https://developer.paypal.com/demo/checkout/#/pattern/vertical

3. You need to decide which type of integration you want to have

a. Client Side Integration


b. Server Side integration ( Recommended)


Reference: https://developer.paypal.com/docs/checkout/how-to/server-integration/

PayPal .NET SDK: https://github.com/paypal/PayPal-NET-SDK

Payments API reference: https://developer.paypal.com/docs/api/payments/v1/