How to build your own simple ChatGPT?

I use ChatGPT daily for my work and it is simply amazing. I have a degree in Computer Science. I have some basic knowledge of ML (studied in CS and went through a few courses on Coursera). I still don't know where to begin. Which topics do I need to cover? Where do I start? I am ready to dedicate months of heavy studying. I want to get how it works. Thanks a lot.