[1.12.2] Custom flat-world generator

Hey there! As the title suggests, I am trying to create my own flat-world generator (lowest block being bedrock, and then 10 layers of obsidian). When I look in the MC Flat world creation, I get even more confused on how to do it. Is there anyone willing to explain me how creating such an incredibly simple thing works? (I don't need code, just pseudo code or an explanation is fine). I am pretty new to modding, so I need to learn stuff I guess.

Thanks for reading!