In this tutorial, you will learn the basics of setting up a new payment gateway with Episerver commerce. This tutorial won't go into any specific payment provider details. Each payment provider that I've worked with has been drastically different.
The aim of this guide is to help you get an understanding of the type of tasks that are required. This guide won't show things like code-first for registering a payment provider with Episerver.
Creating A Payment Gateway
You will need to add the code that talks to the payment provider within a custom payment gateway. I suggest that for each provider you need to integrate with, you create a sperate class library.
Within the library, you will need to define a class, similar to this. Within the process method, you can add your logic to talk to your payment provider.