The great thing about designing for mobile, as opposed to desktop, is that there are a reasonably well-established set of design conventions regarding the application’s navigation.
Furthermore, there is an even more limited number of mobile and tablet navigational design convention that scale well to desktop.
Starting with mobile navigational design is straightforward in comparison to starting with the design of desktop's navigation. The desktop, with its ample screen real-estate, has too many equally good options for how its navigation structure can be designed.
Mobile, on the other hand, with its limited number of pre-established patterns, allows the designer to determine the right solution quickly, and move on. Start with mobile.