I know that the documentation team has been hard at work on some more eCart documentation but currently there is no guide specific to implementing a remote checkout.
You should be able to get it most of the way implemented by using the checkout wizard in eCart. It should prompt you for the values that are needed to submit the transaction to your gateway and then create all the pages for the checkout.
Once you have done that you may need to make use of the response bindings for the checkout to get back details about the order and store them in your db or send them in an email with Universal Email.