Which part do you not understand?
The time/day/month fields? The 'what to run' field?
The field are:
minute hour day_of_month month day_of_week
So, if you did:
00 00 * sh /path/to/script.sh
You would run script.sh every day at midnight.
If you did:
00 00 1 sh /path/to/script.sh
You would run script.sh at midnight on the first of each month.
Does this help any?
-- Jason