This is the account you'll use to make the (fake) payment. Go to https://developer.paypal.com/developer/accounts/create, login with your personnal PayPal account then create a test account.
Create an app here: https://developer.paypal.com/developer/applications/create
Use the generated merchant credentials to set the following variables in API's
Create buyer's credentials on https://developer.paypal.com/developer/accounts/create
And you're ready to go. Use the credentials generated in step 3. to authenticate when ordering.
The button may require multiple clicks to trigger on dev or staging. It should not affect production (see https://github.com/paypal/paypal-checkout/issues/471)