Okay, imagine you and your friends have a special club where you make all the rules and nobody else can tell you what to do. One day, your friend Billy decides that he wants to be the leader of the club instead of you. He doesn't ask nicely or follow the rules you already made, he just tries to take over the club by force. That is kind of like what a coup d'état is.
A coup d'état is when a group of people, usually military leaders, politicians, or other powerful individuals, try to take over a government or political party by force. They might do this because they don't agree with the current leader or they think they would do a better job. But unlike in your club example, a coup d'état often involves violence and can be very dangerous.
A practical handbook for a coup d'état would be like a book that teaches people how to plan and carry out a successful takeover of a government. It would cover things like how to recruit other people to help you, how to make a plan, how to get weapons and other supplies, and how to keep your actions secret from the current leaders.
However, it is not a good thing to promote coups d'état because it often leads to violence and instability in a country. It is important for leaders to be chosen through peaceful means and for people to work together for the good of everyone.