You can't do what you want to do online. The website won't allow it.

You can specify cabin (Coach or First/BusinessFirst) for each individual flight but you cannot specify fare class if your cabins do not match for all flights.

In other words, as soon as you select First/BusinessFirst for any flight in your itinerary, the website will throw errors if you then say "Specify Fare Class" and enter the Coach Fare Classes you are looking for, e.g. "Y, H, K, B, N, O". Only if all segments are in coach can you specify classes in coach. Alternatively, if all segments are in First/BusinessFirst, you can enter "Z, C, J, R, F", etc. and the website will process your request.

To mix & match Coach with First/BusinessFirst on specific fare classes, you have to call CO to do it. Keep in mind that the ticket will be considered purchased via a reservation agent and you will potentially earn 50% EQM (for NW WP) on some fare classes.

-RM