Crazy easy.
Login to your account. Bring up your order. Click the button to return an item.
You'll get a printable mailing label that you can affix to the same packaging you received the product in and send it back. Apple pays the return shipping. You get a refund once they confirm they product has been returned.
Big ticket items (iPads, iPhones, computers) are the same general process but my experience is that you have to initiate the return by speaking to a customer service representative.