How do I make configure a payment gateway for Moneris?

I have a client that would like to sell products via her website. Up until now I've only used MarketPress Lite since all my clients didn't have any existing payment systems. This latest client has an existing account with Moneris and a POS system that she likes. I've done some searching and I've found a couple threads that mention creating a custom gateway for Moneris is possible. However I haven't been able to find any documentation, forums, etc that detail this process.

Could someone point me in the right direction?