When building a new system, you have several options to choose from. You can copy an existing system (depending on its license) like the Simple World-building system, you can use a system generator, or you could start from scratch and have total control.
/Data/systems directory in your Foundry userData folder and rename it from boilerplate-master to your system's name (lowercase, machine-safe).boilerplate and rename them to your system's machine-safe name, such as mysystemname.Boilerplate (case-sensitive) and replace those with a capitalized version of your system name that's still machine-safe, such as MySystemName. These are typically used for the classes in the Javascript files.boilerplate (case-sensitive) and replace those with a lowercase machine-safe version of your system name, such as mysystemname.The generator-foundry package has not yet been updated for Foundry v10. For now, it's advised to instead use the Boilerplate system or League TypeScript Template instead.
You'll need to be comfortable with your terminal/command line and have npm installed. If you are, run the following two commands to install the CLI generator version of Boilerplate (https://www.npmjs.com/package/generator-foundry):
npm install -g yo
npm install -g generator-foundry
Alternatively, you can install it locally to a directory with npm install yo generator-foundry --save and then execute commands with npx instead.
After that, you just need to open your /Data/systems directory in your Foundry userData folder in your terminal and run
yo foundry
to generate your system. The generator will ask you what your system name should be and will provide examples. In addition, it will prompt you to enable optional features such as Active Effects and Item macros, or examples such as ability score modifiers and spell items.
From here, you can skip to the next page or keep reading if you'd like to read about the other options for starting a new system.
If you'd like to see other options for getting started with building a new system to start from, see the Other options page.