To pay using Afterpay, simply select this method out of the options on the payment page. From here you will be directed to Afterpay's external website where you can sign in to your Afterpay account and your payment will be processed.
This means that your payment will be split into four equal fortnightly instalments. Your first instalment will be processed two weeks from the date which you first place your order and you will receive friendly reminder emails from Afterpay prior to the funds been taken each fortnight.
If you request a refund on an order which has been paid for using Afterpay, they will reimburse you for any funds you have already paid and then cancel any outstanding payments. This can sometimes take a few days to process, so if you have an upcoming payment rest assured that it will be refunded if any funds which are taken in the meantime.