You take an Instrument written test, the add-on is 15 questions, you then get around 6 hours of Instrument instruction in Helicopters and you take a check ride. When I did it, I was not current instruments and got both the Instrument and Instrument Instructor done in around 12 hours including a couple of hours for a flight check with the FAA. Its just some differences. Not much and the FAA dose not make a big issue of it. The CFI add-on writtens were 20 questions.