You have to do it from root shell via SSH using the command:
crontab -e
This will throw you into an editor (usually pico) where you can add new entries with the correct format.
For help on format, check:
man crontab
man crontab -S 5
Learn how to edit cpanel crontab. Follow the steps below to complete the task safely.
man crontab
man crontab -S 5
Our support team can walk you through setup, troubleshooting, and account questions.