brew install prebuilt-amiga-dev-tools/prebuilt-amiga-dev-tools/<formula>
Or brew tap prebuilt-amiga-dev-tools/prebuilt-amiga-dev-tools and then brew install <formula>.
Or install via URL (which will not receive updates):
brew install https://raw.githubusercontent.com/prebuilt-amiga-dev-tools/homebrew-prebuilt-amiga-dev-tools/master/Formula/<formula>.rb
brew help, man brew or check Homebrew's documentation.