I want to have vanilla mob drop a custom item

I have tried many methods of making vanilla mobs drop the custom item, but none of them work! I am using mcreator 1.7.5 for mc version 1.11.2, If you have a method that works for that version please answer with the method, btw if you could put it in a step process, like
1. do something

2. do something

3. do something

then that would be great, thanks for your feedback :D!