The full source code and build instructions for creating the images in this repository is available at the Heads git repository and it's Wiki.
Flash needs to be writable first. In case you already run a coreboot-based BIOS, you're fine. If not, the Skulls project might help you getting started.
Their release package
also includes a x230_heads.sh script that you can use to flash these
images.
The Heads firmware images are built reproducibly so anyone can verify the integrity of the images in this repository. Each archive (build) includes a text file with the Heads' git hash that is checked out at build-time and steps to reproduce the exact same image. Verify it by comparing the coreboot.rom hashes.