This is the system I use.
Start off with your plastic ball and a full rack of pins. Aim for the middle arrow (4th arrow, 20th board) throw the ball straight by breaking your wrist back and moving your index finger closer to your middle finger and your pinky away from your ring finger, and try to pick the 10 pin cleanly from the full rack of pins without hitting any other pins. Adjust your feet until you feel that you are standing in the right place to accomplish this on command. Then move your feet 15 boards right, aim for the middle arrow again and throw the ball straight, you should be able to pick the 7 pin cleanly from the rack from this position. It also helps to align your self with the pin you are aiming at, instead of the foul line. If you move your feet 5 boards right of where you shoot the 10 pin clean, you should have the 9 pin and if you move your feet 5 more boards right of that, should be the 8 pin.
For instance I throw a plastic ball straight at all of my spares and use the middle arrow for all of my spares, I will be standing on 37 to shoot a 10 pin, 32 for a 9 pin, 27 for an 8 pin and 22 for a 7 pin. The only time I change this is if I leave a washout, then I stand on 35, and aim for the 5th arrow and line up towards the 2 pin so I don't leave the ball to the right.
Like any spare shooting system it relys on you hitting your target and throwing the ball straight, if either of those things go wrong, you are likely to miss.
This takes a lot of practice, so be patient.
Good Luck