The local overlay consists of a few packages not included in offical Gentoo repository.
Create a file under /etc/portage/repos.conf/ and add the following:
[local]
location = /path/to/ebuilds
sync-type = git
sync-uri = https://path/to/github/repo
priority=9999
where location is the path to where the ebuilds are being stored, e.g., you can put it under /usr/local/overlay/local. Then perfom emerge --sync and you are ready to emerge.
To harden your desktop, you may want to create a hybrid profile which inherits both desktop and hardened profile. For detailed instruction, please refer to Gentoo Wiki at https://wiki.gentoo.org/wiki/Profile_(Portage). Sync the repo and switch to the profile
# eselect profile list
...
[54] local:hardened/plasma (dev) *
# eselect profile set 54
Finally run an update to see if the new set of use flags comes into effect.