Online Store Payment[Help Needed]

hi guyz,

pls i need to know this urgent.

i have a running online store that does its payment through Paypal.

what i want to know is how can i validate that the user's order was successful(because fraudsters might view the source of the page and get the return url for Paypal and just enter it into the browser).

i learnt the IPN(Instant Payment Notification) can do it, but i don't know how because i don't even understand the IPN itself not to talk of how it works(and pls to tell me to read from Paypal because have done that and its not helping my situation)