It is usually instructed to introduce new cron jobs through command lines; but I found it easier (with a better control of current cron tasks) to manually edit (in text editor) the user cron file like
Is it dangerous to edit the file in text editor?
The comments in the default file is confusing. The first line says
# DO NOT EDIT THIS FILE - edit the master and reinstall.
But the fourth line says
# Edit this file to introduce tasks to be run by cron.