A 64-bit machine: cmake for Android is only available in 64-bit
Python 3.5+: to run the build script
Java Development Kit (JDK) 8: To compile Magisk Manager and sign zips
C compiler (Unix only): To build zipadjust. Windows users can use the pre-built zipadjust.exe
Android SDK: ANDROID_HOME environment variable should point to the Android SDK folder
Android NDK: Install NDK via sdkmanager, or via Android SDK Manager in Android Studio

Instructions and Notes

The easiest way to setup a working environment is to open Magisk Manager with Android Studio. The IDE will download required components and construct the environment for you. Don't forget to set ANDROID_HOME environment variable to the SDK path.
Windows users: while installing Python 3 on Windows, allow the installer to add Python to PATH, or you'll have to add it manually afterwards. By default, the Python executable is setup as python, not python3 like most Unix environment. If you have both Python 2 and Python 3 installed, you'll have to deal with the executable name and PATH yourself. To double check the Python version, call python --version.
To run the script, on Windows call python build.py [args...]; on Unix call python3 build.py [args...], or simply ./build.py [args...]. To see the built-in help message, call the script with -h as an argument. The -h option also works for each supported actions to see the help message for the specific action.
By default, the script will build binaries and Magisk Manager in debug mode, which will enable verbose debugging messages. If you want to build Magisk Manager in release mode (through the flag --release), you will need to place your Java Keystore file in release_signature.jks to sign Magisk Manager's APK. For more information, check out Google's Official Documentation.
The python build script uses ANSI color codes to change the color of the terminal output. For Windows, this only works on Windows 10, as previous Windows console do not support them. If you use an older Windows version, a quick Google search should provide many workarounds.

License

Magisk, including all subprojects (git submodule) is free software:
you can redistribute it and/or modify it under the terms of the 
GNU General Public License as published by the Free Software Foundation, 
either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.

Credits

MagiskManager (java)

All contributors and translators

MagiskSU (jni/su)

MagiskPolicy (jni/magiskpolicy)

MagiskHide (jni/magiskhide)

resetprop (jni/resetprop)

SELinux (jni/selinux)

Maintained by many developers in SELinux project

ndk-compression (jni/ndk-compression)

Each library has its own copyright message in corresponding directories

ndk-busybox (jni/busybox)

Makefile for NDK, generated by ndk-busybox-kitchen: Copyright 2017, John Wu (@topjohnwu)
Patches for NDK: Many contributors along the way, all placed in osm0sis/android-busybox-ndk
The copyright message for busybox should be included in its own directory

Others Not Mentioned

Languages

C++ 36.6%

Kotlin 30.4%

Rust 16.4%

Java 9%

Shell 4.9%

Other 2.6%