First, you must put a Pokémon Emerald (US) ROM in the root directory of the repository and name it `baserom.gba`. It should have a SHA1 checksum of `f3ae088181bf583e55daf962a92bb46f4f1d07b7`. 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.
build.sh
install.sh PATH_OF_POKEEMERALD_DIRECTORY
Then in the pokeemerald directory, build the tools.
Then get the compiled tools from https://github.com/pret/pokeruby-tools. Copy the `tools/` folder over the `tools/` folder in your pokeemerald directory.