Mapping Custom Question on Custom Checkout Page

Hi there! C&P is currently working on a new custom check-out page for us and have included a new Custom Question regarding Membership Rewards associated with donations of certain amount and above. I want to map this new custom question to a Custom Field on our Opportunities that we have been using on our current pages. Since the Custom Question is not in our current C&P use, where would I find the information for mapping the new Custom Question and the responses for that question to a current field on our Opportunities? We have been testing the form and would like to verify this as well before we make the new page live.