Custom commerce connector

Is there any documentation on how to develop a commerce connector for an eCommerce platform where AEM may not have reference implementation?

I mainly want to know how to develop the product importer in terms of the classes to extend, interfaces to implement and general guidelines on what the code is supposed to look like?

Is this info available anywhere? Since there are few ootb connectors I would suspect there should be some standard guidelines which the commerce vendors used to implement the connectors.

Thanks.