What is the actual nuclear launch protocol?

One now-cliche question American voters have heard in this and previous presidential elections: “Who would you trust with the nuclear codes?” Donald Trump has been on the defensive recently about this question from his rival Hillary Clinton. Even members of his own party, like MSNBC host Joe Scarborough, as well as fifty GOP national security officials, have expressed concern over Trump having access to the nuclear arsenal. Trump and his campaign have denied this is an issue. But how does the decision making process to deploy nuclear weapons really go down?