[Step 0: have all the build requirements listed in "Hacking on Atom Core". Particularly: Node and npm, and a Python 2.6, 2.7 or 3.5+ executable, on your system PATH. Various packages are needed, including for Linux g++ and gcc (there is a longer list at Hacking on Atom Core), for macOS you need XCode or the "command line tools" package, for Windows you need a valid install of Visual Studio/Visual Studio Build Tools with the right plugins. Recommend getting it via installing the windows-build-tools
npm package globally, as described in "Hacking on Atom Core."]
git clone https://github.com/atom/atom
cd atom
script/build