The instructions for building coreboot on https://puri.sm/coreboot have been up-to-date and accurate for quite some time. I’m not sure why they didn’t work for you when you tried to follow them. Just to be sure I’ve been testing that script on brand new installs over the past few weeks and each time it worked as expected.
That said, the above page is the place to get those documents, the world of coreboot is a fast-moving world so older blog posts (or older posts on this forum) are not the best place to get the most up-to-date documentation on flashing coreboot.