FLY BY THE SEAT OF YOUR PANTS

To fly by the seat of your pants means to do something difficult without the necessary experience and/or ability.

For example, in the movie "Catch Me If You Can," Leonardo DiCaprio plays a con artist who artfully impersonates an airline pilot, a lawyer, and a doctor. However, once he realizes that a life is at risk because he is only flying by the seat of his pants when pretending to the be a doctor, he decides to quit.