On: First THINK C Program (2020)

Where's a good place to find a guide to that?

The definitive reference is Apple's own Inside Macintosh. Many parts of that are still available on the Apple developer web site, e.g.

https://developer.apple.com/library/archive/documentation/ma...

https://developer.apple.com/library/archive/documentation/ma...

For a more directed and more C-oriented walkthrough, the Macintosh C Programming Primer was a pretty common go-to. It's been out of print for ages, of course, but there's a PDF available at:

https://vintageapple.org/macprogramming/pdf/Macintosh_C_Prog...

You can search for "Toolbox" and "Tool Box" here:

Vintage Macintosh Programming Book Library - https://vintageapple.org/macprogramming/