As more of an introduction, the following chapter of TidyText on topic modeling introduces this idea,
A kaggle tutorial:
https://www.kaggle.com/rtatman/nlp-in-r-topic-modelling
A datacamp tutorial
Ideally, you'll have samples you've pre-classified. That way you can measure the effectiveness of various methods and -- if this step was done carefully -- infer the relative strength of these methods on new data.