Another key V8 feature, totals are the car combat version of Mortal Kombat's FINISH HIM! battle cry! When you bring a vehicle's energy down to zero, it will stall and you'll be given a short period of time to deal a finishing blow. The best way to ensure they stall is to get them close to being wrecked, then use the machine gun to take off the remaiing energy.
If you have any Special Weapons in stock, they're usually the best option as a single click will fire and (if hit) total your enemy. In the absence of Special Weapons, you'll have to rely on the regular weapons - specifically any of their charge-up/combo moves. Since the charge-up moves can take time to prepare, the combo is usually the best option. Of the regular weapons, the mortar is often too slow - by the time it hits, the Total window will be closed and the vehicle will already be wrecked. The quickest options are either the cannon (buckshot in particular as it's shells spread out) or the rocket stampede.
It's also worth noting that you don't always need to bring a vehicle to a wrecked state - if the last hit you make on them is with a special/charge-up/combo, this will count as a Total, though from expereince, this isn't always reliable...