What are the app supposed to do, what is its goal and what need is it supposed o fulfill?
Describe how the app are going to be used. A method for this is using user cases and user stories. Sometimes a mockup of the interface is drawn up in this step too.
Shorter than a use case. Are about one to two sentences.
Find the objects in the stories and name them so that they can become classes later on. For this you may draw an conceptual model if needed based on the use cases/user stories from the previous step.
Start by picking out the nouns in all the cases/stories and make a list of them. Then start to weed out duplicates and similar words.
Describe how the classes interact with each other. You can create a sequence diagram here if needed.
Draw an UML with all the classes you need showing the connections.