This is a well explained and concise guide on theory and implementation behind it.

But like many things, there's no replacement for getting your hands on a real life system and gaining real life intuition.

The best way to learn this? Get/make a test bench with safe-ish motor and encoder system. Play with values and measure at the performance in the system. Increasing the P value gains, makes the system "stiffer" but at the expense of stability. Put your hand on the flywheel and "fight it" safely. You can feel the I (Integral) value winding up against your steady state errors. It's quite magical.

As you start to chase the highest performance for your system, you start to think about alternative control strategies. Eventually you will dream up the concept of feedforward control loop. Try to automate the the picking of PID values, then you start to learn about Ziegler–Nichols tuning method. Gain Scheduling. Non Linear Control theory. It never stops.

(If you do any of this, please make sure the motor/flywheel won't kill you if it goes unstable, and please wire a E-stop that's easy to access when it's unstable)