Hi,
Not sure how much you know but does this help.
http://www.viart.com/recurring_payments.html
Maybe not if you are ctually asking how to set up the script rather than what the syntax should be.
But I reckon Viart might load a cron script in for you if you are desperate.
I use putty.exe to log into my server ....then I type
crontab -e
then enter the syntax as shown but you need to use the same file structure as your server uses rather than the example if this is different.
Hope this helps but I googled around and found the right answers for me.
Dave