Start with high level DFD and then break further
1. The top level DFD should be Context driven showing external user interaction with the whole system/product across top level/external trust boundry. 2. The next level break up may focus on one specific use case or feature 3. The lowest level DFD should be preapred only if required that may show all the components within same trust boundry.
Option 1 – Top Down approach: You may take top down approach to start with the whole system further detailed down by specific use cases or features
Option 2: Bottom Up Approach: Start with specific feature/data element and anslyse bottom to top, finding all the entry-exit points in appliaction that influence the respective feature/data element. This may be used if you want to first focus on highly critical functions/data elements first. For example if your application handles Credit Card data of your customers then you may start by analysing all touch points in your application that are only relevant to CC data.
Note: DFD is not a Flow Chart or Class Diagram or a Sequence Diagram.
Read Threat Modeling - Practice Guide to understand how to use DFD during Threat Modeling exercise.