Libreboot is available via binary distributions that make it easy to install (which is what I cover below) as well as source code distributions at http://libreboot.org/#releases. To get the latest binary release, go to http://libreboot.org/docs/release.html, and be sure to download both the .xz as well as the corresponding .xv.sig file, such as:
Once you download the files, use gpg --verify to validate that the signature matches:
$ gpg --verify libreboot_bin.tar.xz.sig libreboot_bin.tar.xz
gpg: Signature made Tue 14 Oct 2014 09:07:32 PM PDT using
↪RSA key ID 656F212E
gpg: Good signature from "Libreboot Releases (signing key)
gpg: WARNING: This key is not certified with a trusted signature!
gpg: There is no indication that the signature belongs