Checkout processes are usually fairly complicated things, but each one has a very similar set of core steps, such as a header basket summary and some full checkout pages (basket, address and payment for example), often with some sort of summary of the basket visible on all the full checkout pages.
Of the common steps of a checkout process which would be the best place to display a voucher code field, when considering the ease of use and feedback for the user as well as the potential effect on conversions while also assuming any entered value is easily remembered between pages