To “know your stuff” means you will be able to combine your knowledge of everything you need and mix them up to create your own “originals”.
“Knowing something inside out”. You must have spent a considerable amount of time learning about the genre you are interested in.
A very large amount of input through a long period of time, accompanied with internalising the knowledge and skills, lead to the ability to make magic with them when the right topic shows itself.
Clear Word Drops 