To run it all your need to know is put it in your filesystem as "/init" and then add this to your kernel command line for the binary you want nit to pivot to after bringing the system up:
nit.target=/path/to/binary
That's it. Minimum viable init for single application appliance/embedded linux use cases.
nit and your target binary are the only things you actually need to have in your CPIO root filesystem. Can be empty otherwise.