The first step is to place the water, salt, garlic salt, dry pasta, butter, and dry mustard into your Instant Pot.
Put the lid on your Instant Pot and press the Manual button. Make sure you use the high setting. You want to set the timer for four minutes.
Quick release when the four minutes are up. When you see the pin drop, remove the lid from the Instant pot.
You want the Instant Pot on low heat, and then add half of your cheeses plus the evaporated milk.
Stirring this up will allow the cheeses to melt. Once the first part of the cheeses have melted, add in the second half of the cheeses.
Once all of the cheese has melted, season as desired, such as adding more salt or some pepper.
Let the mac and cheese rest after removing it from the Instant Pot for about five minutes before eating.