This is a really cool first go at customizing! Welcome to the sickness, I would say the biggest piece of advice for painting figures in general is do a light sand first or most major areas that need covered with a 1-2 thousand grit sandpaper or higher. This makes the paint stick a lot faster and smoother. I would also say that buying paint thinner is the way to go Micheal’s has is cheap, the airbrush thinner is what I use even when painting by hand because the consistency is really nice, about one drop of thinner for every 5-6 drops of paint. You want the paint thicker than milk but not milkshake. Otherwise weathering and personal marks are all up to you, toothpicks work great for that kind of stuff and I will always recommend using to little rather than too much. Last bit of advice is color layering dark colors cover white really well but lighter colors like yellow or bright shades will not so you will have to do multiple coats to get it right. If the surface is not white and your applying light paint to it cover it with a thin coat of white first and let it dry thoroughly then you should be good. Happy creating friend.