Don't start working on this if you have deadlines or important work. Legacy Macs on newer OSes cam be unpredictable in some cases so please be prepared with backups such as a native version of macOS for your hardware.
[CRUCIAL]KNOW YOUR HARDWARE
Your Mac's SMBIOS
Your CPU name and generation
[CRUCIAL]A BASIC KNOWLEDGE OF COMMAND LINES AND HOW TO USE A TERMINAL/COMMAND PROMPT
This is not just [CRUCIAL], this is the basis of this whole guide. We can't help you if you don't know how to cd to a directory or delete a file.
[CRUCIAL] A machine that is compatible as seen in the Compatibility section.