Setup New Payment Module..simple..but need hep

[alinux]

Hi
I am setting up a new payment module, the payment procedure is simple, a file.php takes all the necessary variables and sends those to a payment gateway..after that the result is redirected to a page I specify.
I have tested it without virtuemart and it works fine. However, I have no clue how to do it in virtuemart.
Any A-Z howto's around or sample code someone has done on this before ?
What concerns me most is that once I redirect the user to the payment gateway and then I get the response, I need to insert into virtuemart that the procedure has succeeded, so what would that query be. Another thing is that once I leave from virtumart to the payment gateway and then back, how can I make sure that the user is displayed his own payment page and not one for someone else ?
Regards

[alinux]

Anyone please