Building a system in Foundry is a lot of work, but often necessary to support new and niche games. First though, check the official package repository for systems - it's both easier and better to collaborate with others than make many diverging implementations of a system.
Once you've confirmed you want to write a new system, here are some good options:
¶ Boilerplate System
URL: https://github.com/asacolips-projects/boilerplate
The Boilerplate System is a lightweight starter set that includes a small number of examples (such as character ability scores) and a few helper CSS classes for laying out your sheets without actually having to write new CSS. It also includes the Sass source files I used to the compile the CSS if you're comfortable with NPM.
¶ Starting from scratch
You can also start completely from scratch! This is the most advanced option since you need to already know what to build, but more information can be found at https://foundryvtt.com/article/system-development/.
- Prev: Getting an empty system together
- Next: Stuff to be aware of