I would caution against system definition and system design being put as a singular task. Definition should define what the system does, what tasks it performs, the way the user interacts with the system. System design is more focused on the how, with a leaning toward psychology and flow.
I specialise in system documentation. While I have overseen design aspects, this is a highly specialised task. If you accept this premise, I would recommend the following approach:
1. Discovery - review what you have now; notes, sketches, powerpoint, etc.
2. High level view -- user journey (focus on user experience and outcomes), components, non-functional requirements
3. Functional Requirements - Features, Use Cases, Candidate data model
4. Testing Plan
5. Acceptance criteria
Have a great day further,
Dumisani