Getting rid of chewy chicken

No instructions are going to be that precise; there are too many variables at play. What temperature was the chicken before you started? Exactly how big are the pieces? How precisely can you measure and maintain the temperature of the oven or cooking medium?

What you need to be sure that your chicken is perfectly done is an instant read thermometer. At first, aim for 165F (74C), that's the minimum "safe" temperature. After that, you may choose to go a little higher, especially for dark meat. Some people prefer a few degrees lower, especially for breasts. Whether temperatures under 165F are safe is a bit controversial, you need to assess any risk on your own. Once you know the exact temperature that seems perfect for you, that's where to aim. The temperature will continue to rise after you remove the chicken from the heat. For small pieces that "carry-over" temperature rise will be pretty negligible; a whole chicken will rise a few degrees.

Whatever recipe you follow, take the temperature a few minutes before the time in the recipe. As you gain experience, you'll get better at zeroing in on doneness by appearance.

To tenderise chicken or any meat, you can consider soaking it in the juice of a raw papaya or in pineapple juice. The former is preferable because it is more effective and has a more neutral taste.

Both these things act as tenderisers for animal protein.