Asking early and in writing is the safest approach.
Find the contact details in your confirmation email or on the official refund policy page. Write with your order number, name, email used and the date. State plainly that you are requesting a refund under the 60-day guarantee. Ask for written confirmation and any return instructions.
Save your message, any reply and the date. If you get no reply, follow up in writing before the window closes. As a last resort, your card issuer can handle a dispute, which works best with a clear paper trail.