NoSQL databases approach modelling from a different point of view generally. NoSQL modelling usually begins with asking questions on how the data in the database is going to be queried, so the main modelling is 'what are the list of questions that I want to ask of my database', whereas with SQL databases, modelling usually is a function of what data you have, that is, what answers I have already. This mental exercise of finding out what questions one would like to ask of the NoSQL database serve to define the entities and the relationship amongst those entities.
Once you have a list of questions, you then need to design a model that will answer those questions via the NoSQL database. To model it, I typically use Creately which is an online tool for visually representing models of various kinds.
Another method could be to represent your NoSQL model in JSON format and then use the wonderful D3.js javascript library to create a data-driven document. See this website for more details and the templates available to visually represent your model.
Hope it helps.
