From a “good first issue” to a substantial PR, I will tell you how I have on-boarded the elm-in-elm open source project (https://github.com/elm-in-elm/compiler).

Following my experience step-by-step, I will explain how I have:
* simplified some tests,
* easily added a new feature in a large unknown code base,
* detected a bug, while all seemed good,
* improved the way the compiler can help the developer,
* helped new contributors to be involved in this project.

Moreover, since this project is a compiler I will also
interleave explanations about the compilation process...
This will be a chance to realize that while such software is
complex, it is nonetheless definitively accessible.

In short, this talk will speak about development methods, tooling, compilation, how to be involved in an open source project... and the Elm language!