I had a really difficult time finding resources I could understand on how to make a mod as someone with no prior experience with them or coding. I have a hard time learning things that aren't outright explained to me so this is my attempt to help those like me.
I use JSON Editor Online to make all of my .json files its super easy to use and does point out errors in your json.
You can validate your .json files before ever launching the game by using the SMAPI Json Validator. To find it go as if you were parsing a log but then on the right hand side under tools there is a json validator option.