I don't think there is any sure-fire method to teach gunnery in any plane (at least I've never heard of any method that works every time for everyone), but here are some tips that might help.
Convergence must be set to a range that you feel comfortable shooting at. Where's batfink? Look at the quote in his sig and it explains it very well. You don't want to shoot at your convergence range, you want to shoot at the range that you're comfortable at and have the most success at. So if you feel comfortable shooting at 400 yards and have the most success there then set your convergence at 400. If you feel comfortable shooting at 200 yards and have the most success there then set your convergence at 200. Trust me, this works!! I used to have my 109's set to 450 because I thought convergence didn't matter in them. Then one day I changed my 109F guns to 300 just on a whim to see what happens because that's where I feel most comfortable shooting at and have the most success. I went from having to put many bursts into my opponent to kill him to having to put only 1 burst into them to kill them. I instantly went and changed all my planes convergence to 300 after that. It works!
Get close. Ok, now get closer! This is more of a tip than an absolute thing you must do like convergence settings. It's a good idea to get as close to your opponent as possible before shooting. Why? Well it takes less time for the bullets to reach the target, meaning you have to lead the plane a lot less. You bullets will do more damage as well. Some bullets like the 303's on the spitfires lose a lot of their hitting power at longer ranges (I don't know exactly where it starts dropping off significantly, but I'd guess somewhere around 400-450 yards). So, if you get within 200 yards of your opponent before you start shooting then you don't have to lead him as much and your shots will do maximum damage. If you do do this and get in close then remember to set your convergence for this range as well for reasons explained above.
Of course, you do want to be shooting at ranges you feel comfortable with and have the most success at, so if you like engaging targets at 400 yards then set your convergence to 400 yards. If you're still having trouble hitting the target even in your comfort zone then try the 2nd tip. Set your convergence somewhere in 200-300 yards and get as close to your opponent as you can before pulling the trigger. It's easier to aim since you don't have to lead them as much.