Hi Priyanka,
firstly think about the heart of app its domain model,then go with brain of app its microflows nextly go with the parts of body its pages which you showcase your data the whole combinely made an application and prepare the document in the same format of heart,brain and body parts.
Hi, Priysnka
If the application has workflow then you can look into it, and you can write the document on it, it will cover technical things.