Much the same as developing any programs:
- Define the problem you need to solve
- Design a solution to that problem
- Implement the solution
- Test it
- Ship it!
- Maintain it.
There will, of course, be iterations among those steps.
The big difference between embedded microcontroller projects (including STM32) and "general" programming is that your problem & solution are not purely software - they will include hardware (electronics, etc).
A complex system that works is invariably found to have evolved from a simple system that worked.
A complex system designed from scratch never works and cannot be patched up to make it work.
