Background

10/20/09 - Initial guide written by CC
10/21/09 - Zen added to portage tree, had to make revisions to this

Forewarning

     The Zen kernel is not supported by Gentoo. Do not report problems with the Zen kernel to them. Use at your own risk!

Options to obtain Zen on Gentoo

Zen is in the portage tree! (10/21/09) - This means to obtain it, all that needs to be done is "emerge zen-sources", and of course keywording it. Obtaining the kernel with git can be read the GIT Based Installation/Management guide.

Adding zen-sources to package.keywords and installing

Zen is currently located in portage and is keyworded by ~ARCH. If you are CRAZY and running ACCEPT_KEYWORDS="~ARCH", you can skip step 2.
1. Sync the portage tree to ensure you get the latest version (legacy versions are also available in the tree, if needed mask the newer version in package.mask)

sudo emerge --sync

2. Add zen-sources to package.keywords for whatever architecture you are running (example: ~x86, ~amd64, ~ppc, etc.)

sudo echo "sys-kernel/zen-sources ~amd64" >> /etc/portage/package.keywords # For amd64 architecture

3. Install the sources, they will install to /usr/src where you can configure and compile them

sudo emerge zen-sources