Expressions people spontaneously make

I wonder how people come with their own expressions not in idiom dictionaries.

I didn't gather what I've encountered before, so I'm listing just one example.

On another forum there was a new member asking a question about how to play a game. He said "This is my first time posting on here, so try not to kill the fresh meat, k?"

'kill the fresh meat'...I get the meaning, but I couldn't find the phrase in any of the idiom dictionaries I have, so I'm assuming he made it.

So...what am I supposed to be able to make such creative expressions? Reading, listening, speaking and whatnot is the answer I'm expecting, but I don't seem to have become more creative after years of

doing them.