You're talking about having a 'custom payment method' which only works for credit card processing and extending that 'custom payment method' to allow bank account processing...
There's no way I or anybody else can help you 'extend' a 3rd party payment method unless you tell us which one you're using, and then still it's a 3rd party method.
The blog-post by Teresa describes how you can 'hook-up' Sitefinity to any 3rd party method, once there you're basically on your own (and in hands of the 3rd party API).
Achworks for instance provides an API guide
and HTML form examples
on how to implement ACH (creditcard or check) payments on their respective website...