Start with deciding, if you want to use Matlab's tool GUIDE for creating the GUI or if you program it by your own. Due to some pitfalls and incompatibilities I prefer the programmatical approach, but many other use GUIDE.
In both cases the GUI is created at first with some decisions: Where should which element occur? How should the GUI look after resizing? How are the data imported and the results exported? Such questions can be solved with a pen and paper efficiently.
After the elements have been implemented, it is time to program the callbacks - the functions which are evaluated, when the GUI elements are activated. Here the "communication" between the callbacks is not trivial. Serach for "share data between callbacks" in this forum.
If you decide to create the dialog programmatically, you find valuable instructions here: https://www.mathworks.com/matlabcentral/fileexchange/24861-41-complete-gui-examples
