Firstly, take a Monterey cheese, grate it properly, and then set it aside. In America, the cheese usually comes in slices, so it becomes impossible to grate. So you have two options. You can either chop it using a food processor, or you can simply cut it into small pieces using a sharp knife.
Now, take milk in a saucepan and then heat it over medium heat till you find it simmering slightly. Make sure that you do not boil it at any cost.
After the milk starts to simmer, add all the cheeses you had grated.
Now reduce the heat to the lowest flame in the gas.
Then keep on stirring it. You will find they the cheese will slowly start to melt.
Even after that, do not stop but keep on stirring with the help of a flat spatula or spoon to check if the bottom of it burns.
After about 20 minutes, you will find that the cheese sauce will be fully melted.
Finally, turn off the flame or heat and then let the sauce cool a bit. Because in this way, it will allow it to thicken up a bit while you serve it.