In Rakefile, add this line to the top:
Run crontab -e to access crontab editor
* * * * * cd /path/to/rails_app; bin/rake some_task
Change the timing, path to your rails app and the rake task that you want to execute.
Read more: http://tutorials.jumpstartlab.com/topics/systems/automation.html
(note: in the tutorial, the author suggests to use full path to rake. I tried but it failed so I just used bin/rake instead)