First, you must put a Pokémon Mystery Dungeon Red Rescue Team 1.0 (US) ROM in the root directory of the repository and name it `baserom.gba`. It should have a SHA1 checksum of `9f4cfc5b5f4859d17169a485462e977c7aac2b89`. Then, follow the OS-specific instructions below.
Make sure that there is an environment variable called DEVKITARM with the path of the directory before the "bin" directory containing "arm-none-eabi-as", "arm-none-eabi-cpp", "arm-none-eabi-ld" and "arm-none-eabi-objcopy".
Then get the compiler from https://github.com/pret/agbcc and run the following commands.