Create an account on https://www.paypal.com.
Go to https://developer.paypal.com, and login with the account created previously.
In the Application tab, click on Sandbox accounts, and then Create Account.
Create a test account for the client.
Fill the form, picking the Personal (buyer account) choice for Account type.
Create a test account for the merchant.
Fill the form, picking the Business (merchant account) choice for Account type.
Click on the merchant's account and then on Profile. You'll find all informations needed to setup the seller account
on the Btop application.
Create an application : In the My apps tab, click on Create and manage Classic API apps.
Click on New app, fill the form and submit it.
Your application keys are now available.
Check that the Paypal payment plugin is deployed by going to the Configuration > Plugins page.
In Btop backend, click on Configuration > Sellers, and then Add a seller.
Fill in the seller's name.
Check the Activate checkbox.
Associate the Paypal payment plugin to this seller.
The plugin isn't configured and shows as "Not ready". Click on the gear icon to configure it.
Fill in the form using information from the 6th and 9th steps of Paypal's conguration, and click the confirm icon.
Your Paypal account is operational and ready for use in your application payment.