Set cron jobs

Hi everyone, I am using centos 64 bit which is running 2 wordpress sites,
In that sites I am using WP-Robot plugins which is used to do autoposting,

For that I want to set cron jobs, I don't know how to do it,
I am having ssh access but no cpanel,
How do I set the cron jobs for regular 6 hrs as interval?

:)

Login to the user for that site and do:
crontab -e
Then add something like this:
* */6 * * * php -f /path/to/file.php
Save & exit the editor to complete the update.

Login to the user for that site and do:
crontab -e
Then add something like this:
* */6 * * * php -f /path/to/file.php
Save & exit the editor to complete the update.

Hey
I am doing something like this,
In my VPS I have two sites, and only one ssh login user for both,
after accessing ssh, in root directory I typed
crontab -e
But it comes as no crontab for username - using an empty one

Further I didn't added anything, am I doing right?
and in this code:
* */6 * * * php -f /path/to/file.php

what does path to the php file means? which File ?

Thanks for your reply