This is fairly easy to do, you will just need to start by finding the OEM CAN high and low wires. They are in a few spots, but the D43 junction connector is the easiest place to find them in my opinion. It is basically a CAN hub for all the devices in the car that use CAN to function.
Here is the diagram.
From there you will take the OEM chassis CAN high and low and run them to the correct pins on the Haltech ECU. I like to use the main connectors for OEM CAN and the auxiliary connector for Haltech CAN
The main connector CAN pins are listed below:
B24 is CAN low
B23 is CAN high
Once that is done you will need to tell the ECU you are using CAN for the FRS / BRZ. You can do that in the "Main Setup" tab, under the "Devices" section. There are 2 protocols to pick from, the auto climate control and the manual climate control, pick the one that matches the chassis you are using.
Once that is done, go through the list on the screen shot above and select each function you plan to use and assign the input as CAN instead of analog.
From there you can test that they are all working and you should be good to go!