"We applied and were approved through the USCIS back in March, but I've heard nothing from the NVC yet."
From our experience, once you get the approval you should be getting the instructions after you receive the NOA2, which is a Notice of Action indicating approval of your application. But if you have not heard anything, we highly recommend you call the National Visa Center or check your status online.
Currently, the NVC website tells you to wait a minimum of 60 days because that's how long it's taking to actually process the NVC packets. That includes your affidavit of support, your civil documents, etc. We've heard so many cases where customers will just wait forever, and just expect things to happen. That's not the case.
The NVC phone number is (603) 334-0700.