Motivation:

You need to quickly capture and analyze requirements for building a prototype or proof of concept for an enterprise system.

Suggestion:

1. Capture any artifacts related to a business need. They may be some emails, presentation slides, a proposal document, a legacy system, some similar systems, a feature list or an initial requirement specification.
2. Define terminologies (terms), build a background and context around the terminologies, expand the terminologies into business roles, business workflows, business components, business entities (objects), and business artifacts as much as possible.
3. Identify the main stakeholders (roles) who will interact with the system (e.g. Guest, Admin, Developer, User, etc.) and their corresponding business problems or needs (i.e. their motivation of using the software system), and their current business workflows. Create as many scenarios (real world or domain business workflows) as possible. Create a domain model (a model of real world or domain entities, their properties and their relationships) if necessary.
4. If there is an existing or similar system then identify the core components that each stakeholder will interact with (e.g. Accounts, Profiles, Reporting, APIs, Lessons, Videos, etc.) and their corresponding purposes.
5. Identify the main tasks that each stakeholder will perform (e.g. Register an Account, Log in System, Create/Edit Profile, Create a Bucket, Generate an API Key, Create/Edit/Delete a Lesson, Create/Edit/Delete a Video, View Bandwidth, etc.).
6. Identify the inputs and outputs and create mock-ups (sketches) or capture similar screens for each task. The quickest way to create a mock-up is to find similar existing screens in your system or external systems, and modify them. You can also search for templates of similar features in the Internet, and modify them. You can also brainstorm a  new one if you are good at creativity and imagining.
7. Build or draw a prototype demonstrating 2 or 3 critical end-to-end workflows (An end-to-end workflow is a sequence of tasks that solves a real world problem completely). Contrast a current business workflows with the end-to-end workflows demonstrated by the prototype. The quickest way to create a prototype is to find similar existing workflow of of your system or external systems, and modify them. You can also repurpose a business workflow for a prototype. You can also brainstorm a  new one if you are good at creativity and imagining.
   
8. Implement a proof of concept related to the 2 or 3 critical end-to-end workflows.
9. For a complicated system, you may need to implement a proof of concept, build a prototype, and create a work break down structure or product backlog in parallel to ensure that the requirements are technologically and economically feasible.