How I approach a new GUI design? [closed]

The context:

Summarizing, the family business relies on proprietary software that is used to manage client files, budgets, etc ... But this represents a significant annual financial burden, it is not entirely tailor-made (non-used features) and the implementation of new features are done under budget.

I have some experience coding in C++ and Python, I've never designed a GUI, and when trying to draw a "draf" I just stare at the screen and nothing comes up.

The question:

When facing a project, what process should I follow to design a GUI? (It will be a in-house desktop App)

Note:

Also, the software they use does the job pretty well with the interface that provides. I've though of just "Copying" the layout but, just thinking about it felt bad... It is a bad idea or it's totally fine? (Should I post a photo?)