I am trying to install a new payment processor. Right now, visitors to my site only have an option to give their credit card, but I also want them to have the option to do a "Bank Account Transfer." How would I do this?
Unless William has addressed your "Bank Transfer" question, and you actually mean you want users to pay by entering their bank details, often known as ACH transfer.
You will need a merchant that allows that, an example post on this: https://civicrm.org/blogs/adixon/iats-next-generation-payment-processor-acheft
The link William provided indicate IATS and Vanco offer ACH,
You can find information about setting up payment processors in the documentation: Payment Processors.
However, a 'bank account transfer' sounds like it would have to be a 'pay later' option. I presume that the user won't be able to do this as part of the transaction on your site but would have to separately go to their bank and make a transfer to your bank? If so, I would suggest that you enable the 'pay later' option and provide your bank details so that people can make the transfer into your account.