Pre-heat the oven to 200°C/Gas mark 6 (reduce 20°C for fan assisted oven). Add the macaroni to the oven dish. Pour the water over the pasta and mix in the cheese sauce. One fully mixed, Place dish into oven for 30 minutes, I like to stir every 10 minutes to make sure all pasta gets coated and fully cooked under the liquid. After 30 minutes (providing all the pasta is soft and fully cooked) sprinkle over the grated cheese and return to the oven until the cheese on top has melted.
Sometimes I fully cook the macaroni ahead of time (and store it in an airtight container in the fridge) and assemble it, then put it in the oven for 10 minutes before sprinkling over the cheese.
To add more delicious flavour, I have cooked bacon, cut it into little pieces and added it before the grated cheese when doing the last mix of the pasta.