Changing my fluorescent lamp to led lamp
Using Ohms law. Since we know the current, source voltage, and LED forward voltage drop, we must calculate for the series Resistor. R = (V Source - V Forward) / I.

(6V - 2V) / 0.02A = 4V / 0.02A = 200Ω

220Ω is the next resistor up.

Now since we have the resistance, we can calculate Wattage of the Resistor. P = V (of Resistor) * I

4v * 0.02A = 0.08W or less than 1/8th (0.125) Watts. A 1/8W Resistor would work.

A better solution is two leds in series, sharing the current.

(6 - 2 - 2) / 0.02 = 100Ω2v * 0.02A = 0.04W. A 100Ω 1/8W would work.