First thing is first, make sure you have WinRAR and PaintNET installed. Once you have those installed, go to his website and download your truck of choice. Once the file downloads, open it up. This is what you'll see.

Next, you'll want to open the Open Definition File that is highlighted in the previous picture. Once you do so, your screen should look like this :
Next, open these folders in order!!
vehicle/truck/upgrade/petejc/companylogo
Once you do so, you should see the .dds files that you can change in PaintNET pictured below for example.

Click on whichever "yourlogo" file you would like. There is black and white options along with different parts such as front steps and rear steps. Just choose one that fits your preference. For this purpose, I'll be using the yourlogo_white_front_steps. Double click on it and should open in PaintNET.


Now that is completed, make a new layer in the bottom right for your logo placement.



































Now go to File/Open and find your custom logo. It'll open in another tab so you'll want to click on your logo and click the CUT button, and paste into the yourlogo_white_front_steps file.


Size your logo to fit the square. DESELECT THE BACKGROUND LAYER!! That way ONLY your logo is showing and not the text underneath.

Hit File/Save As and save your file EXACTLY like pictured below:

Once you saved your file wherever in your desired location, drag and drop it into the Open Definition File you opened at the beginning of this guide. It'll ask you for permission to change the file, hit yes.

Now that you have your logo in the Open Definition, put the Open Definition file in your mods folder. I usually put my SCS files on my desktop first, then navigate to Documents\American Truck Simulator\mod where I would then drag my SCS files in my mod folder. Now go in game and activate your mods in your mod manager and BE SURE to load them in the PROPER ORDER!!
Here you can see the Open Def file is ABOVE the truck.

If all steps were completed, you should now be able to go into your truck customization and select the part you created your logo for and find it under whichever “yourlogo” you’ve chosen in the beginning.