Have you read the FAQ (section 3) about this and looked at the [JavaScript Syntax I made for adding things like this](
https://github.com/morepurplemorebe...Sheet/tree/master/additional content syntax)? Because that is intended as the place to start, especially if you have no coding experience.
Basically what you do is fill out the syntax example files with your own information. Then, when your script is done, copy it into the "Add Custom Script" dialogue. Any script you put there will be run every time you open the sheet. So if you add a class, that class will always be available in the sheet unless you reset the sheet or open the "Add Custom Script" dialogue and remove the script.
I'm happy to take a look at your script if it produces errors. If you don't want to spend your time writing the scripts, I can also make them for you, if you are willing to compensate me for my time.