1. Head over to https://developer.paypal.com/
2. Log in with your PayPal account
3. Hover over your name in the top right and click on Dashboard. Click the Create App button
4. Enter in "Web" for the App Name and make sure your account is selected for the Sandbox developer account. Click Create App to create the app.
5. You'll be taken to the page that shows the details for that app. Switch to the Live tab at the top
6. Click Show under the secret to reveal the secret key
7. Copy the Client ID and Secret