This article assumes that you have already rhc client installed on your local machine. If it's not, you can read about it here.
1. Adding crontab cartridge to you application
Login into openshift and choose your application. Click on "Or, see the entire list of cartridges you can add" link and add Cron cartridge.
2. Creating folder for backups on server
$ rhc domain show
and find Git URL line, e.g.:
Git URL: ssh://email@example.com/~/git/myapp.git/
Use only inner part of URL for ssh connection and type:
$ ssh firstname.lastname@example.org
$ mkdir $OPENSHIFT_DATA...