It doesn't have to be all the kingdoms, you can focus on the one you know best.A general overview would suffice also. Like after the last western empire who took power and how. Why was the empire so fractured that so many cultures and languages developed in this small area is what i'm wondering.
I partially explained some of the reasons to which the "compact" size of Western European nation-states is attributed to in modern development theory in this answer from last week. However, the juice of your question asking for an overview of the emergence of any one of the western european nation states, apart from being somewhat trivia-seeking, really does involve a lot of work and might not get a complete answer.