You can add any loyalty points you have earned from previous orders at stage 4 of the checkout process. If you have points in your account you will see a tick box at stage 4 when selecting payment method.
To use your points simply tick the box next to 'Use my reward points ...'. You should then select your payment method and click 'Continue' to proceed with payment and order completion.
If you wish to check how many points you have before you checkout you can do so by going to the 'Reward Points' tab on your account page.