Adding other payment gateways in ProSites .

I was trying to add some payment gateways in prosites. Due to lack of developer documentations and confusion in code, I am losing my time in adding additional gateways in prosites checkout. Lot of confusions and waste of time due to lack of documentations. Can anyone provide me the process ? Where to hook for settings / where to process the payment in order to work similarly as current available gateways ( paypal, stripe ) does in Prosites.