Expanding the world bottom inside a Custom dimension

I recently found and followed a tutorial ( https://mcreator.net/forum/90778/how-make-world-deeper ) On how to make a world deeper, I followed what I learned and tried to do the same inside my custom dimension. When I try to generate a world in minecraft, It immediently Crashes. The user said it requires more advanced coding. I have some knowledge with coding and I'm sure I can get it working if I knew what to do. Anybody who knows, Please help!