Should you practices the noble eightfold path all at once or master is separately? I read in a book while I'm studying the path, that you should practice the path all at once. It can't be separate. That seem to me like a very large task. Is this true.
When you first learn to drive a car, do you learn how to start the car on one day, how to accelerate on the second day, how to brake on the third day, how to turn the steering wheel to the left on the fourth day and how to turn the steering wheel to the right on the fifth day? No. It doesn't make sense. You have to learn and develop these steps altogether in tandem, and practise them together.

Another example is this. When you learn to cook, would you learn to cut vegetables on one day, then learn how to wash ingredients on another day, then learn to boil on another day, then learn to simmer on another day and so on? No. Instead, you learn and practise them altogether at once by trying to cook a certain recipe. By practising them together, you slowly deepen your cooking skills. You would then expand to add frying, broiling, baking, steaming, poaching, sauteeing etc. to your techniques, but you would practise them together in one recipe.

Similarly, in the case of the Noble Eightfold Path, you would have to first cultivate Right View (the forerunner of the path) by learning the Buddha's teachings (the Dhamma). Then you would start developing Right Resolve, and decide to take refuge in the Buddha, Dhamma and Sangha, and practise the five precepts (which form the core part of the virtues - Right Action, Right Speech and Right Livelihood).

As you deepen your progress into Right View by studying more of the Dhamma, you then increase your knowledge of the virtues (Right Action, Right Speech and Right Livelihood) and try to practise them with more fervour. That's more of Right Resolve and Right Effort being applied.

As you deepen your contemplation of the teachings, you start cultivating Right Mindfulness and Right Concentration, and then go into the meditation practices. These meditation practices would then lead you to better understanding of the Dhamma through first hand experience, which is improving the Right View once again. If you stumble into the five hindrances during meditation, you may need to deepen your practice of the virtues once again. This of course again is applying Right Resolve and Right Effort towards making progress in meditation and cultivation of wisdom.

So, every step helps the other. You can't practise only one of them at a time. But of course, learning the Dhamma and practising virtues tend to come before meditation, in general. This is just as in the case where you need to start a car and accelerate, before braking, but you would anyway do them in one session of driving, with repetition of all the steps as needed, to take you to your destination.

Even if you have not tried to meditate, if you continually reflect and ponder upon the teachings (that you have studied) and see how they match your experiences in life, you would still begin to understand the teachings from first hand experience. Then meditation could still progressively come later.