The coupon field is on the cart page.
Before checking out you can review the bottles in your cart. To get there, click the cart button at the top right-hand corner of the Different Drop website.
On the cart page, you will see the items in your cart listed and a coupon field below that. Enter your coupon code and click apply. And that's it! Happy shopping :)
NB: some coupons only work if you are logged into your account. You can log into your account at the link below.