Recompile NSIS script

As you say you have no experience of NSIS Script then I think you are jumping in at the deep end and need to start by learning about the methods used for creating PortableApps compatible versions of software.

I suggest you do a bit of research/learning using: https://portableapps.com/development and especially the sections on the Launcher/Launcher Manual, the PortableApps Template and the PortableApps Installer. However, if you want to find out more about the NSIS Scripting language the best place to start is the NSIS website.

For the majority of apps out there you will never have to even look at NSIS to make them portable using the PortableApps format (I've been a developer here for 3+ years, I've looked at NSIS, but have never actually needed to use it). You would only need to do something with NSIS if there is a specific situation that the PortableApps Launcher can't handle.

Along with checking the developer documentation as mjashby mentioned, you are also welcome to tell us exactly what needs to be done for whichever app you are trying to make portable, and we can then help you figure out if NSIS is even necessary for it, and if not, how to do what you need just with launcher options.

You say you "downloaded a portable app, and extracted the launcher, in order to get to the script".
Could you possibly give a little more detail? i.e. What app did you do this to? How did you "extract" the script from the launcher? and also, what exactly was your goal in doing this?

Note: I could try guessing (I'm usually fairly decent at it with this sort of thing), but I feel we'll get things straightened out better & sooner for all of us if you answer these questions.