UML is used during all stages of development to help keep your developers and clients on the same page as to what the project will end up being.
one of the worst feelings is going to the customer with a product and being told thats not even close to what they originally asked for. or that you need to extend the dead line because the mvp is going to take longer then you thought.
with UML you are able to keep your devs and clients on the same page from start to finish to make sure that your project stays on track with what they want.
there are so many sites out there that will walk you through how to draw UML Diagrams. one that I found that makes it easy is https://creately.com/diagram-examples
they have examples and templates for creating
Interactive Site Maps
Just to name a few. They have several different organizational boards or patterns you can use the templates to help you create one that will work for you.
UML's are a great way to keep things organized and to show non dev's exactly how things are going to work so they can understand why certain things are going to take longer then others. and they can see the flow of the project.