Move Redmine plugin directory.

$ cd /path/to/redmine/plugins/

Clone plugin from github.

$ git clone https://github.com/ngyuki/redmine_redcarpet_viewer.git

Install dependency by bundler.

$ cd /path/to/redmine/
$ bundle install

Restart Redmine.

$ sudo service httpd restart

This Redmine plugin is based off the original soruce code of Toshiyuki Goto forked from https://github.com/ngyuki/redmine_redcarpet_viewer.