Here's how I installed them (enter into a terminal one line at a time). Do this at your own risk. My understanding is that downloading from a repository is simply a matter of trusting the source, as theoretically it could be malicious software:
What this does is add the repository to download from. Then you install the tweak tool (you should be able to find this under "system settings" in Elementary OS. You need this to be able to set the icons). Then you installed the icon set, which you can now find under "tweaks" in system settings.