This is something I put together a while ago - IIRC it covers EasyTherm 4.0:
Now, here's how to use EasyTherm with MS1/Extra. When you open EasyTherm, there will be several options you need to set. It defaults to GM sensor calibrations. On our ready-made units, you will leave the bias resistor value set at 2490 ohms; you'll only change this if you have opened the Megasquirt and swapped out the resistors.
The boxes in the middle of EasyTherm are where you enter the sensor calibration data. You will need the sensor resistance at three temperatures. You can obtain this from a factory service manual or simply by testing the sensors in boiling water, ice water, and water at a measured room temperature. The sensor needs to be what's known as a negative temperature coefficient thermistor. This is simply a fancy engineering word for saying the resistance goes down as the temperature goes up.
Next, you need to select the code version. You'll set it to "Custom" for MSnS-E firmware such as 029v. Leave the start address alone. Then you get this information into the code by clicking "Write .inc and .s19 Files." It will prompt you to open an S19 file containing the code. Select the file that corresponds to the MSnS-E version you are using. If you do not have a copy of the MSnS-E firmware, you can download it here:http://www.msextra.com/manuals/MS_Extra ... _Index.htm
EasyTherm will generate four files. One is an S19 file that will have the same name as the original with "_mod" added to the end of the file name. The other three are .INC files for MegaTune. Their names are airdenfactor.inc, matfactor.inc, and thermfactor.inc.
EasyTherm can load the S19 file into Megasquirt, too. Power down the Megasquirt, open it up, and put the boot jumper in place. Then connect it to the computer and power it up, ideally on a Stimulator. If you power it up on a car, disconnect Megasquirt from any coils or ignition modules it may be connected to. Then specify the COM port where you have Megasquirt attached and click the "Download to MegaSquirt" button. EasyTherm will prompt you to select the S19 file you need; it will be the one that ends in "_mod" that you've just created. It will then download your newly calibrated firmware. Shut down the Megasquirt, remove the boot jumper, and close its case back up.
If you had the firmware you started with in the same folder as your car project, it will automatically put the INC files in the right place. Otherwise, you'll need to copy them so that MegaTune will be able to correctly interpret the sensor data. The default location for these files is in the EasyTherm folder if you have altered a firmware copy in the EasyTherm folder. They go in the mtCfg folder in your project folder. If the project was named car1 and you have the usual folder setup for MegaSquirt, for example, you would want to copy the INC files to this folder:
Once you've got that taken care of, you will have your firmware uploaded and MegaTune will know what to expect. This takes care of your sensor calibration.