Start by opening the crontab config file as the user you would like to execute the script as (or sudo for root)
sudo crontab -e
then, all you have to do is to add a line with this syntax to the end of the file:
<Minute> <Hour> <Day_of_Month> <Month> <Day_of_Week> <Year> <command> <args>
where time parameters is a number corresponding to a specific time to match, i.e set the day of month to 1 to execute every first day of the month. Use * to represent any time in the choosen parameter or 0 for the very first occurrence of the time, i.e:
0 0 * * * command
to execute the command at midnight once every day. Good luck!