Download the plugin and extract it into the plugins directory.

Go to the plugins directory and run :

bundle install --without development test RAILS_ENV=production

If you need to migrate the database then run the following command :

rake redmine:plugins:migrate RAILS_ENV=production

Now restart redmine :

touch /path/to/redmineDir/tmp/restart.txt

You should now be able to see the plugin list in Administration -> Plugins and configure the newly installed plugin

Uninstalling the plugins :

rake redmine:plugins:migrate NAME=plugin_name VERSION=0 RAILS_ENV=production

Remove the plugins directory and restart redmine.