I found this quite easy to follow: https://www.buildyourownlisp.com/ for building a not-quite-Scheme in C. I didn't get massively far but only because of the sheer amount of other shiny things.

Yes I have seen that one! It's on my list of resources. There's also this, which I have been studying the code of as I follow along with Crafting Interpreters, to try and incrementally understand the codebase:

https://github.com/vibhavp/skeem/blob/master/src/builtins.c#...

The simplicity of the eval function is so cool!