When logged in you'll notice a new payment method has appeared on the checkout page - Pay With Points. When selected, this will convert your available points into a £ coupon. Simply then click on PAY NOW for the points coupon to be applied against your purchase. You may need to tick the T&C box first!
If your points total doesnt cover the full order amount, you can the pay the remaining balance with your payment card or Paypal account. Once your new order is confirmed, don't forget that you'll also receive some new points to set against future purchases.