Start with a purpose
An embedded system is a computing system built into a device to perform a particular function. A temperature display, a washing-machine controller and a sensor logger all have inputs to interpret and outputs to control. The interesting part is the relationship between computation and the physical world.
Consider a temperature display. The sensor produces a reading, firmware interprets it and a display presents the result. Power, wiring and timing all matter even if the calculation itself is small.
Separate the building blocks
Draw the power source, controller, input and output before choosing a board. Each connection has an electrical interface: it might be an analog voltage, a digital pin or a communication bus. A working program cannot compensate for an incompatible signal level.
Firmware gives these blocks behavior. It may configure a peripheral, acquire a reading, check whether it is valid and update the output. When behavior is wrong, inspect these steps individually.
Learn through observable changes
Start with one output, then one input, then combine them. The first useful question is not whether the code looks sophisticated; it is whether the observed output matches the expected one. Keep a short table of changes and results.