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
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)
PayPal .NET SDK: https://github.com/paypal/PayPal-NET-SDK
Payments API reference: https://developer.paypal.com/docs/api/payments/v1/