Ask HN: GUI Design. Where to start?

I study Computer Engineering and I currently work on my final degree project. I develop software for a touchscreen tablet which will be integrated into a vehicle to be a part of its electronics system.

So far I implemented almost all the necessary functions and met the functional requierements. My current GUI is just a rough draft and has no structure. I have 6 weeks to design an automotive GUI and I am not sure where to start.

Can anyone give some advice? What software would use to make a screen design? I develop a Qt Quick Application using Qt Creator and made my simple GUI using QML.