A Digital River Javascript library is used to render a checkout modal to take users through the checkout process. You need to update your commercetools cart and payment with the data required by the integration and then hand it over to the Javascript library. Refer to Authorize a payment for more detail.
Data is mapped from commercetools resources to Digital River resources to allow for successful sharing and synchronization of data. For more information on how this is done, refer to Map shared data used in the integration topic.