Hello there,
I'm data designer, since my graduation as enginner in information system, i realized several diagrams, the most i use is UML (Unified Modeling Language)
In your case we gone use Classe diagrams to know the data you will manage, an activity diagram to know the global workflow of your scenarios and others diagrams according to the BRD document. By the way i use PowerAmc Software to design diagrams but i can use others.
I assure that I can satisfy you completely and want to have a long term collaboration with you and i will be glad to offer you my services. Please do not hesitate to contact me, i will be glad to work with you.
Best Regards!