Easiest way to get you started
Master Arm on
Pinky switch to Radar (Aim7)
Once within 5nm of the bad guy Point COD Aim thingy at the bad guy (or good guy; if you're a bad guy)
Cage Mode
Jester Context Button Hold
1 Po-tae-to... 2 Po.. -> 4 Po-tae-to
Pull Trigger
Evil Laugh optional, but highly encouraged
Better still, don't do any of this and do the training mission which shows you an other method altogether
nikoel's post in How to a fire a Sparrow in a dogfight? was marked as the answer