Developing a mobile app, the agile way
So as a website, developing a mobile app is a real challenge. When you draft your specification document, always look for the main features that your app will include and try to identify the different components/features that you have a high chance to struggle with.
Once it is done, ask yourself how difficult they are to implement within your app. If some of them are just impossible to set, just ask yourself how you could pivot your project and/or if you project can still continue.
Once done, when you start working, just focus on those features and try to deliver them at specific milestone. Note that within a mobile app, the design is the easiest part to do so you can easily keep this part for later. As far as your features are working you are on the right track.
Here is a very inspirational picture, well known among agile trainers:
The idea is to consider that you should not wait step 5 in order to get the feedback from your client. It is better to ship it after your first feature is released. Like this she/he is already able to give you one feedback that you can use in order to improve this feature or move to the second feature and so on and so forth. By prioritizing your work and by delivering several steps of an achieved app you are avoiding delays and to get a client which is finally not that happy about your final result.