How To Create A Proper .json File For A Custom Minecraft .jar?

I am working on a custom Minecraft client as a personal project, not anything commercial, and I want to try launching it from the launcher. I am working on it in Eclipse, my Java IDE, and it runs fine in there. I exported the client folder as a jar from there, and then got stuck. I know I can't launch it without a proper json, but the problem that I am having is that I don't know what to put into it. I know I name the jar and json files the same as I name the folder, but with their respective file endings, and that I put the files into the folder in the versions folder. I have tried looking stuff up and I have found solutions that have worked for other people, such as deleting these lines from the 1.12 json:

among other solutions that worked for others, but not me. Minecraft-crafting I really want to finally get the json to work, and I don't know how I would get it set up properly. What would be the changes necessary for my modded .jar to work? is there any way to launch without a jar? I have been at this for days, so any help is very appreciated!