Follow these steps:
Download the plugin and place it in the installdir/apps/artifactory/artifactory_home/etc/plugins directory.
Restart JFrog Artifactory Open Source.
$ sudo installdir/ctlscript.sh restart tomcat
So long as the plugin file name ends in .groovy, JFrog Artifactory Open Source will automatically load it on startup. For a complete list of available plugins, visit the JFrog Github repository.