I recommend starting with a simple "Hello World" mod in Fabric or Forge to get familiar with the development environment and the API. Then, gradually add features from your MCreator mod, focusing on the custom grass block and bonemeal interactions first. Attempt to extend your MCreator mod with custom Java code. This is very complex and often leads to instability. It involves exporting your MCreator code and trying to integrate it with a Java modding project. Not recommended unless you're already comfortable with Java.