thumbs, do tell.
btw looks great!
To understand what resistor needs to be added, you need to understand how an LED works.
For a conventional lamp, the current in the circuit will be determined by the resistance of the lamp, and the voltage you apply to it. It's ohms law. I=E/R
Also, a conventional lamp will drop all the source Voltage. If you connect the lamp to 12 V, the lamp will drop 12 V. Even if you connect a 6 V lamp to a 12 V source, the 6 V lamp will drop all 12 volts. It won't last long, because you are exceeding it's design limitations, but it will drop the 12 V.
An LED is a totally different animal. While it does drop a voltage, that voltage is fixed, regardless of what voltage you connect to it. Not only that, but an LED has no real resistance when on, behaving much like a closed switch or piece of wire.
It is for that reason that whenever you connect an LED, you must add a resistor in series. Without a resistor the LED will act like a short circuit, and the current will be high. The LED will burn out, the fuse will blow, or both. The resistor limits (and determines) the current through the LED.
To determine the resistor size, you must know some things about the LED's you will use, and how those LED's are connected. LED's vary a little, but most LEDs drop between 2 and 3 volts, and need a current of 15 to 20 mA to be nice and bright but not burn out.
For my flashlight, I actually connected the batteries and measured the LED voltage drop and current using a digital multimeter. Measuring the voltage drop was as easy as putting the meter across an LED while on. Mine measured 3V.