Use the same tools as they did!
I'm going to do my best to not confuse anyone with unexplained technical jargon here. Lua is one of the many programming languages available to developers and is certainly popular among game developers due to its accessibility. One of those developers is Unknown Worlds, the developers behind Natural Selection 2, a first-person shooter and real-time strategy hybrid with a very involved community.
Unknown Worlds created their own Integrated Development Environment, often abbreviated IDE, using Lua for Natural Selection 2 and called it Decoda. An IDE is an application like Microsoft Visual Studio; software that developers use to create games.
So, now that we know all of that, I can finally say that Decoda, Natural Selection 2's IDE, is available to anyone as an open-source application. This will allow anyone to download it for free, change the IDE itself, modify Natural Selection 2, or even use it to create their own games. Got to love a developer that gives back to the community like this.
If that doesn't sound like it is way over your head, you can download Decoda from here.
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