Cheapest and easiest way is the "Military Competency" app. 10 bucks. Go through the question banks and take practice tests until you're consistently in the 80s. Took me a couple days to be ready. Not sure on Shephard air, but I'm guessing it's a bit more than 10 bucks. Also, the FSDO will write you up for basically any rating you can justify. In my career, I've managed to sea-lawyer two FSDOs into writing me an airplane instrument rating, and a CFI-H and CFII-H certificates... all about what paperwork you have and what it says. If they buy it, you get the rating.