After the initial call with the nonprofit and your own research, you should be able to get a general idea of the direction your project will be going in. It's time to define everything: from your problem statement to your users, user pain points, and your schedule.
Crystalizing Ideas into specifics
You will be creating a product requirements document (PRD) in the first 2-3 weeks, which will be reviewed by the directors and other leads as well as your nonprofit. It includes:
The PRD has multiple roles. It communicates to the nonprofit about the product the team promises to deliver and its timeline. It enables the team to understand what the product entails and what the product itself entails, the nuances and details that were discussed during the ideation phase. It documents the non-obvious decisions the team makes so team members can reference back to it. It communicates to the directors/other leads and provides a way for them to give substantial feedback on their proposed product. It also allows the PM/TL to look back at it, during the middle of Execution, to reevaluate product decisions and timelines.