If your chicken breasts are very large, slice them into smaller portions (around 200-250 grams) in weight.
You need a saucepan that is big enough to fit your chicken breasts in, in a single layer. Fill a saucepan with water, so that when you add the chicken they will be covered by at least 2 inches of water.
Bring the water to the boil, add the chicken and pop the lid back on. Remove from the heat.
Let the chicken poach in the hot water without removing the lid for 30 mins. After this time your chicken should be cooked. You can double check the chicken is cooked by checking the internal temperature with a meat thermometer, it is cooked if the temperature is 76 degrees Celsius or above.
If your chicken is not cooked after this time (if your pot did not hold its heat well or if the chicken breasts are large this could occur) reutn the pot to the heat, and simmer until cooked.
Rest the poached chicken on a board for a few minutes before slicing.