depends on your OS, and if you have other coding experience or not (think JavaScript, Python, Perl).
Also, it'd help if you don't mind working in a terminal/shell environment (console).
Otherwise, you'd need to find an IDE (integrated development environment) for your OS and ideally one capable of building a project using the corresponding build system (GNU make/autotools).
If that doesn't sound too scary, it's not that complicated actually.
You'd probably want to check out youtube tutorials/videos covering your OS and the IDE in question, and then build a really simple project from source, to take it from there.
Alternatively, if you have previously tinkered with virtual machines or containers, that might be another option.
To be honest though, it might be better to team up with others interested in atlas, according to the mailing list, some FG users are still contributing to the project, and maybe you can reach out to some of them to see if they'd be interested in working together.
So, if you have never done any coding before, havent' built any programs from source, don't know about source code management, you are unlikely to find the experience very enjoyable - unless of course you have a strong background in related fields (often maths, software, IT, physics, engineering) - or simply have plenty of time on your hands to learn new things, i.e. because you are in high school, retired or under quarantine
Seriously though, this community is rather active actually - and it might even be possible to reach out to some of the folks who have created scripts to build/update FlightGear and related tools - for instance, there is a "scripted compilation" article in the wiki, and if that supports atlas/map, you'd be all set actually (at least on supported platforms). And even if you are using a different OS/platform, that would be a good starting point, e.g. to build/run the software inside a container/VM (docker).
Either way, it's a good way to get in touch with people who have the right background and experience/expertise:
http://wiki.flightgear.org/Scripted_Com ... ian/UbuntuThe script can be seen here:
https://sourceforge.net/p/flightgear/fg ... compile.shAnd a list of contributors can be seen here:
https://sourceforge.net/p/flightgear/fg ... compile.shI am saying this, because those are the people "in the know", i.e. they have a track record of building FlightGear related tools, and automating the process.
Again, depending on your backgrounde/time frame this could be a no brainer, and depending on the status of atlas, it could also be a huge mess.
So it's not for the faint of heart
But it'd definitely be a worthwhile thing to look into. I guess, without any experience, I'd suggest to look at a few youtube tutorials covering how to build a simple "hello world" style app from source (written in C++)
If you have that working, the next step is building an OpenGL based application. And the rest is really just following instructions and updating dependencies as needed, i.e. trial and error.