The normal way is to cut into the positive side, with a high wattage resistor then connected to ground on one of the smaller terminals. This way, you cut the circuit and also dump the output from the alternator to ground to prevent any running on. If you just want to isolate the battery, then I guess either way would work, but if you want to have it as an emergency cut-off to switch off the engine from outside of the car, then I think you need the alternator connection/resistor. There is a diagram on this thread https://www.lotus7.club/forum/techtalk/battery-cut-switch-0