How do I buy credit on Orange?

Hey there, could you please clarify something for me? I'm interested in purchasing credit on Orange, but I'm not entirely sure how to go about it. Could you guide me through the process step-by-step? Is there a specific app or website I need to visit? Do I need to LINK a payment method, like a credit card or bank account? Also, are there any fees associated with buying credit, and how quickly will the credit be available for use? Your help would be much appreciated!