Noob-proof (N)3DS "Custom Firmware"

Luma3DS is a program to patch the system software of (New) Nintendo 3DS handheld consoles "on the fly", adding features (such as per-game language settings and debugging capabilities for developers) and removing restrictions enforced by Nintendo (such as the region lock). It also allows you to run unauthorized ("homebrew") content by removing signature checks.
To use it, you will need a console capable of running homebrew software on the ARM9 processor. We recommend Plailect's guide for details on how to get your system ready.

Since Luma3DS v8.0, Luma3DS has its own in-game menu, triggerable by L+Down+Select (see the release notes).

First you need to clone the repository with: git clone https://github.com/AuroraWright/Luma3DS.git
To compile, you'll need armips and a build of a recent commit of makerom added to your PATH. You'll also need to install firmtool, its README contains installation instructions. You'll also need to update your libctru install, building from the latest commit.
Here are Windows and Linux builds of armips (thanks to who compiled them!) and makerom (thanks @Steveice10!).
Run make and everything should work!
You can find the compiled files in the out folder.

See https://github.com/AuroraWright/Luma3DS/wiki

See https://github.com/AuroraWright/Luma3DS/wiki/Credits

This software is licensed under the terms of the GPLv3.
You can find a copy of the license in the LICENSE.txt file.