Although you can do this with the at command, crontab is better suited for this task as it is repetitive.

crontab -l (ell) will show you your current cron jobs.

Set your editor to vi (setenv EDITOR vi in C-shell, for example).
crontab -e will let you edit the crontab file.

For 2am everyday:
0 2 * * * script_name

Save the file when done. Check your work with crontab -l.