A fuel cutoff switch can be a security device both in the sense of saftety and a theft deterant. When done properly no one will even know it exists except you.
The first step is to locate your fuel relay. In general you will find this underneath the drivers seat. Mine happened to be a black box with a green cap. Using a multimeter/voltagemeter to locate the power wire to the relay. Once you have found it cut it and attach (solder is ideal) two independent wires to the two freshly cut ends. On the opposite end of these attached wires connect them via a switch. Depending on the switch location you can end up having a nice theft detterant. They can’t steal your car if the motor won’t start. I will leave the location up to your creativity. Enjoy and be safe.