Your first step is to talk to John Lewis so he can give you some pointers on how to go at it.
There's lots of fun chances for missteps.
Your BEST bet to start is to use the chromium development environment, and the sources we use, rather than upstream, just to make sure you can do it at all.
I have set up a peppy to only ever load seabios, which was not hard. But I did not start from the code at coreboot.org.
Sorry you had that problem.
ron