The color and typography tokens are extracted directly from Figma’s built-in color and text styles. So if you already have those in your design system then your color and typography tokens are ready to be exported.
The spacing tokens require additional setup but it’s a one-time straight-forward exercise to add spacing tokens to the export operation.
At the moment, the plugin can only export tokens from the Figma file in which its run. The color and text styles must be local styles in that file, and the spacing token representative nodes must be on the active page when the plugin is run.