You will be presented with the Application Management Page. Scroll down to REST API apps and click on Create App.
You will be shown a form where you have to enter Application Name and Sandbox Developer Account. Fill the form with appropriate information. Then Click on Create App.
You will be brought back to Application Management page. Here you can find Client ID and Secret.
Copy these credentials and keep it handy because you will need it in the next step.
Scroll Down to Sandbox App Settings. Fill Callback/Redirect URL from miniOrange Oauth Client plugin which you copied in the last step. Then Click on + or press Enter.
You have successfully completed your Paypal App OAuth Server side configurations.