Add this to /etc/portage/repos.conf/tthanh.
[sk-overlay]
location = /usr/local/overlay/tt-overlay
sync-type = git
sync-uri = https://github.com/tthanh/overlay.git
auto-sync = yes
/usr/local/overlay doesn't exist by default so create it if needed.
[ ! -d /usr/local/overlay ] && mkdir -p /usr/local/overlay
Finally you can update portage.
emerge --sync