Don't understand as when install sentinel
  
  Install the command in the crontab at the end of the file (Replace YOURUSERNAME with your username) :

  crontab -e
  Push enter again
   * * * * * cd /home/YOURUSERNAME/sentinel && ./venv/bin/python bin/sentinel.py >/dev/null 2>&1

 I can do crontab -e, open one file, but what mean "push enter again"?
enter last line " ***** cd/ ....", and save ? that is all?