Step 3: Creating and testing the circuit
Note the red and black cables in the power cord, these are the power and ground cables, the only two you will need to mess with.
While the cable is plugged into the computer, take a voltage reading, mine was 5v. Not sure if this is standard but I'm guessing it is for most keyboards that simply run off computer power.
So with 5v as a power source, and given that each LED has a forward voltage of 3.3v while using 20mA, then a resistor of about 100ohm should be just right.
It isn't entirely necessary to use one resistor per LED but I did just in case although thinking back it is definitely overkill. You could always use one resistor for the whole circuit considering the LEDs are wired in parallel, to make it easier on yourself.
After you find the correct resistor, create the circuit on a breadboard first and then get some wire and power it by putting the wires into the receptacle of the black and red wires of the keyboard cable while it is plugged into the computer. Just make sure your circuit works with the supplied voltage basically...