Need a Linux Kernel Expert to teach me

I’m trying to learn Linux and I need help with my raspberry pi 4 I bought.

I have a expansion board i’m trying to connect to my raspberry pi 4. I need someone to tutor me on how to add a device tree overlay.

https://www.raspberrypi.org/documentation/configuration/device-tree.md

I’m trying to follow the tutorial above but I don’t get it. *You must be knowledgable enough to answer my questions