First, login as admin, and go in Settings. Go in the Finances tab, and make sure Tool used for subscriptions is PayPal.

First you will need to create developer account on PayPal on the following link. After you create the account open the Dashboard from the menu in the right upper corner.

Now select My Apps and credentials and click on the button Create App.

Now fill the informations that are required and make sure that you will select the Merchant App Type. After you created the app you will see the Client ID and Secret credentials generated. Copy them and fill your PayPal credentials in Settings-> Finances. First, test in development - sandbox mode.

Create account on the official PayPal website and the login with the account.

Open this URL and make sure that you are logged in before you open this URL.

You will see the option for creating new plans. Create your billing plans and then fill the plans information in your admin panel (Pricing plans).