For the injection part just use process hacker 2. then find the minecraft process and inject into javaw.exe by right clicking it and finding the inject dll option(If i remember correctly)
then for the dll part.
You need to learn how the java native interface works.
Then just create your basic dll project. write the code in it, inject into mc, you have hacks
I recommend youtube.com/watch?v=Go4BBQnkB4I&list=PL36bJ4YMRalhyCwLMb3EOqK9 oTvOeFdvY