Determine the voltage flowing into the circuit. You can find this either on the power supply of a device or the battery. Most circuits use 12 volts.
Find the electric current required for your electrical circuit. You can find this on the circuit or in the manual of the devise you are going to power. The number is written in amperes.
Divide the voltage by the electric current to find the resistance, which is the ohms the resistor needs to be. For example, if you have a 12-volt circuit requiring 0.015 amperes of current, you will need a resistor with 333 ohms (12V / 0.015A = 0.333).