This tutorial depends on Lean, VS Code, and mathlib. You can install them following the instructions in the mathlib repository, https://github.com/leanprover-community/mathlib.
To use this tutorial, you need to set up a project folder. Open a terminal and type:
leanproject get mathematics_in_lean
Then open the project in VS Code:
code mathematics_in_lean
Once VS Code starts, open the file welcome.lean.
That will load the tutorial in a
separate window, and you are good to go.
PRs and issues should be opened at the upstream source repository.