Everything starts with the case file number; MNL******** . Once you have your case file number is when you can start to move forward with the actual visa application.
For the Fiancee, K-1 visa you will get your MNL# from NVC after they receive and review your case file that was forwarded from USCIS after your petition was approved. This can take from 2 weeks to a month. Once you have your MNL# it is time to pay your visa fee, setup your Account/Profile at USTRAVELDOCS, complete the DS-160, then schedule your interview.
For the Immigrant visa, which most times I will refer as the spousal, IR/CR-1 visa. All immigrants visas applications process the same with very few exceptions. You will get notice from NVC upon CASE COMPLETE, NVC will email and/or mail a letter with your interview date and other relative information.