Under the My Cart or Checkout Page, you may type your voucher code into the empty field box and click "Apply".
To remove, just clicks the delete button. 

Please note that the promo code cannot be combined with other cart promotions/ only allowed to redeemed once.  Furthermore, it is important to note that each of the promo codes will have its validity (especially campaign's promo code).

Therefore, the promo code will not be able to be applied once it's expired.  

Once the order is cancelled, you may no longer reuse the promo code anymore.