There are few methods to do the polarization curve. Since you are not using potentiostat, you need to have different resistors with wide range lets say 10 Ohm to 10000 Ohm. For this you can use resistor box. While doing polarization curve, let the reactors remain in constant OCV, then you connect resistos starting from high to low, in each resistor you record the stable value from multimeter while your resistor still in cirucit. At last you will get voltage and current. The latter one by Ohms law. Then you plot current vs voltage curve and simultaneously power curve.