In the illustration above it is described how the IR receiver should be connected and how the kill switch circuit can be implemented using a optocoupler. The idea of the circuit is that the optocoupler will be connected to the logic of the motorcontroller and as soon as the kill switch signal goes low the motors should stop.
The components in the circuit are the following.
VDD: This is the voltage source that is used to power the optocoupler, should be same as logic high on the motorcontroller.
R2: This is a pull down resistor.
R1: Is a serial resistor which limits the current through the diode.