Determine current. Current is the flow of electricity measured in amps. For example a current has four amps in the circuit.
Determine voltage. Voltage is the difference in electrical potential from two points, measured in volts. For example, there is two-hundred volts in a circuit.
Divide voltage by current to calculate resistance. Resistance is measured in ohms. In the example, two-hundred amps divided by four amps equals fifty ohms.