What I did
Total Mortgages already had a well set up style guide, which they use for their website and other branding materials. I adapted it to fit the needs of a mobile app.
One of the main things I focussed on was to create a clear navigational system, so that users can quickly learn how to interact with the app. I established a visual structure and pattern for main pages, secondary pages and popups, which stays consistent throughout the app; so users can easily anticipate the way the app will function.