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

1.
layman -a multilib-portage

2.
emerge -av1 portage

remember that multilib-portage is following the 2.2* branch of portage,
so it only has testing keywords

3.
Check the target of /etc/portage/make.profile:
readlink /etc/portage/make.profile

Replace it with a mini custom profile. If the original make.profile
symlink was relative, you may need to prepend an extra “../” to it
because you will now be referencing it from a subdirectory of the
original reference:
rm /etc/portage/make.profile
mkdir /etc/portage/make.profile
echo /path/to/current/profile >> /etc/portage/make.profile/parent
echo /path/to/multilib-portage/profiles/base >> /etc/portage/make.profile/parent

4.
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-portage
[multilib-portage]
eclass-overrides = gentoo multilib-portage

5.

then either recompile world:

emerge -e world

or use /path/to/multilib-overlay/bin/add_multilib_abi 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