You just need to carry out an act of purchase as if you were a customer.
To test the Lemon Way for E-commerce solution, we have provided test credit cards with the following numbers:
Type of card: VISA or MASTERCARD
Date: Before 2030
CVV Cryptogram: 000
Advice: Make sure that Opencart and our Lemon Way for E-commerce management interface is in test mode. You can find the management interface here: https://ecommerce.lemonway.com