Is it possible to edit a custom script without having to set up new script projects from scratch?
I've only tried messing with scripts once before and no matter how slowly it was explained to me it all went over my head. I realise this is not a good start.

Basically I have a custom object in my game from a CC creator that I want to try something out with but it would require me to edit the script he created. I know how to VIEW the script using ILSpy but is there any way for me to make quick and simple alterations to it? (not with ILSpy obviously)
I don't want to make a new script, I want to edit the existing one already in use by the custom object.

In short: no, you can't just take an already existing script and build on top of it. However, you can create your own script that inherits an already existing object's script. You can't modify any of the existing code, but you can add stuff to it which would be able to change the output. If you want to make minor alterations to a script that exists, and I'll get a lot of flack for this but there isn't really another way, you're going to have to copy and paste the already existing script into a new one and modify that.