bash# crontab -e
Note that the default editor is vi. If you are not familiar with vi take a quick look at Section 4.5.2. Each line is a command to be run. The command is made up of five numbers and a command. The numbers are minutes, hours, days of week, days of month, and months. If you wanted the command /sbin/reboot to run every Sunday morning at 3:30 then you would have a line like this:
30 3 0 * * /sbin/reboot
When it does not matter what the value is you can put an asterisk there instead. This does not mean that minutes should be the asterisk if you do not care. You need to specify or you might have the program running every minute.