The main thing to remember about this model is it is constantly changing. Even if you are doing nothing, your comfort zone is shrinking. So in order to expand your comfort zone, look for doable stretches that provide a challenge but aren't too overwhelming.
Also make sure you don't spend too long in the stretch zone as that leads to burn out over time. You do need to factor in recovery time in the comfort zone so you can reflect on your experience.
For example, if you're learning to drive, it helps to gradually build up the time behind the wheel incrementally. Interspersing your driving lessons with the 'comfort zone' of being a passenger when others drive. This allows you to observe other confident drivers which can help you process your own experiences.