Using the board look at all of the squares controlled by:
– a knight on c3
– a knight on f3
– a knight on c6
– a knight on f6
Then, without the use of the board repeat the exercise again visualizing the squares controlled by the knight on f3, c6 and f6.
Perform this exercise for two sessions or until you feel comfortable. You should also repeat exercise #2 until you are extremely comfortable performing it.
Remember it is important to build upon fundamentals, and exercise # 2 is an important building block in developing chess visualization skills.