First, if you want to install and use it via layman:

layman -a multilib
cd $PORTDIR_OVERLAY/layman/multilib ($PORTDIR_OVERLAY is for most 
probably /usr/local/portage)
git checkout -b portage-multilib origin/portage-multilib

After that, every layman sync should update the branch as normal, 
switching is done with

git checkout <branch>

the main branches are master and portage-multilib

A basic way to use this branch:

First, install the modified portage version:

1.
emerge -av1 portage

remember that multilib-portage is following the 2.2* branch of portage,
so it only has testing keywords and is hardmasked by package.mask

2.
check the target of /etc/make.profile
rm /etc/make.profile
mkdir /etc/make.profile
echo /path/to/current/profile >> /etc/make.profile/parent
echo /path/to/multilib-overlay/profiles/base >> /etc/make.profile/parent

3.
let the multilib overlay eclasses override the main tree ones: add the 
following to /etc/portage/repos.conf or create it with the following
content:

[DEFAULT]
eclass-overrides = gentoo multilib
[multilib]
eclass-overrides = gentoo multilib

4.

then either recompile world:

emerge -e world

or use /path/to/multilib-overlay/bin/add_lib32 to add the useflag
to the database. In this case, you might have to run
/usr/bin/lafilefixer once since multilib-portage does remove most of
.la files during install stage