The IAC will be controlled by the ECU.
Setup depends what IAC valve you have but unless it’s an open/closed variant it’s likely a stepper valve, and you create a curve based on temperature- that’s an open loop. A closed loop is more complicated as it constantly adjusts the IAC valve to try and maintain an certain idle speed.
You need to find out what IAC valve you have and get the calibration specs from the manufacturer or I guess, set the calibration in tunerstudio while looking at the valve or with the engine running so you can see when it’s fully closed and not allowing air to leak in. Then you can start building your curve to allow the air leak based on the engine temp. Of course you’ll need to make sure that your engine temp sensor is accurate too.
https://www.diyautotune.com/support/tech/other/idle-tuning-megasquirt/