To total a car you must hit it with your special or with a charge-up attack when the vehicle is stalled, Or in other words is just about to blow up when its on Fire, When this happens quickly press and hold one of the buttons for a charge up and when the 3 little bars fill up release the charge up attack to finish the enemy (EDIT, you don`t need three bars, one will do, Thanks to DoctorHilarius), Now to make this easier you could do this in a custom match set it to 7 enemies and pick the weakest cars for the enemys so you get them fast. You only get a few seconds to do this this attack as the enemy car will blow up after about 10seconds.