2.1 volts at 20 milliamps is all you need. I am going to assume that you don't know how to hook this up so let's start from scratch. The LED wants to see 2.1 volts, but let us say you want to connect it in your car. I believe that is around 13.6 volts. This is where you use a resistor in series with one of the led legs. 13.6 volts minus the 2.1 volts is 11.5. Now use ohms law 11.5 divided by the 20ma or .020 equals 570. This is the value of resistor to use. (find something close like a 560 or something)