when you are in the garage, go to car customizations.
go to the bottom of the list where it says special abilities and pick the EMP. you will see some empty looking stars with a bar below. the bar tracks your progress for each star level.
while either cruisin around or racing, try to avoid hitting anything in your path, and eventually you will build up the little circle in the bottom right corner of your screen which will then light up 1, 2, or all 3 thirds in a blue light. once thats lit up just hit the left bumper and you will activate the special ability. the more you use it, the faster you level up.