There are basically two steps that go in a cycle. Old words from the English language are taken, and they are changed in how they sound and what they mean, making a slang, sometimes slowly sometimes quickly the slang terms become excepted as words by society and eventually they become the words being changed. If you think about it the language changes based on what we find to be the way the popular people talk. In the past it was the rich or the actors/esses in plays that started a change in culture, now it are the musicians and actors/esses. That is what I think about it, there are probably more ways of explaining it, if I even answered that the way you wanted.
