When you are ready to check out you can select Alipay from the list of payment options. After you have selected Alipay you can scroll down to the check out page then click on "Confirm & Pay".
Alipay is available to download from Google Play and Apple's APP Store. Once downloaded you can follow the steps to activate Alipay. Then you can use either the Alipay APP or web browser to pay for your order.