Clone the repo and run install.sh as root from within a Gentoo live environment.
$ ./install.sh --help
Usage: ./install.sh [OPTION]...
OPTIONs:
-s, --stable Use a stable profile
-d, --dev Use a developer profile
-g, --generic Install can be booted into and run on any computer
-v, --version Print the installer version number and exit
-h, --help Display this help message and exit