Well, first with product recommendations. Many people (including myself) have found great results with the "Meguiar's Twins" - M105 (compound) and M205 (polish). These 2 products alone are capable of correcting some serious defects when paired with the proper pads, and they also finish down very nicely, leaving a gorgeous glossy finish behind. M105 can be hard to work with, as it dries pretty quickly, dusts quite a bit, and can be hard to remove - I've read that M101 is more user friendly and still works just as well as M105, although I have not personally used it.
Here is a write up I did for a 2008 G8 GT that was severely swirled up... I was able to correct all of the swirls with only M205 and a White Polishing pad.
2008 G8 GT - One Step Correction
I used both M105 and M205 on this 2010 BMW, as it had some more severe defects. You will notice that I was able to fully correct 2000 grit sanding marks with M105 very, very easily in this one.
2010 BMW 335i - DISO Mess!
As far as needing an old hood to practice on, I would not consider that totally necessary. DA machines like the PC7424XP and GG are EXTREMELY safe, and even with an aggressive compound like M105, the chances of you causing damage are very slim. With that being said, if you feel more comfortable practicing on a scrap hood first, go for it!
The general rule of thumb is to first
prime your pad before you being polishing. This will ensure your pad is properly lubricated before touching the vehicle's surface. Next, apply 2-3 pea sized drops to the pad and then polish your section. Continue to apply 2-3 pea sized drops before each section and you should be fine. I try to
clean my pad on the fly after each panel, sometimes more frequently if I am working on a particularly rough vehicle. This will ensure that any spent product or clear coat on the pad will be minimized so that you can continue to work clean. After cleaning your pad, simply add 2-3 more drops of product and continue your polishing routine.
As far as contoured panels, just do your best. While it is best to keep the pad as flat as possible, there are certain times when you must use it on a curved surface, such as a fender. The best practice, however, is to try to avoid buffing directly on any sharp body lines or edges, as these tend to have thinner paint. As I said, DA machines are very safe, so the likelihood of causing permanent damage is slim, but more likely on these types of edges compared to a flat panel. For tricky areas, it is best to have a smaller set of pads. I, for example, have a set of 3" pads that allow me to get to tighter areas that my 5.5" pads would have trouble with.
Just remember, this isn't rocket science. Machine polishing is becoming more and more beginner friendly and is just something that takes practice and patience to master. There is an endless amount of information here on Autogeek Online, so you'll never be without an answer.
Hope this helps. Please let me, or anyone else, know if we can help to answer any other questions!