a basic setup, you need a 100 ohm 1/2watt resistor, a 0-1k ohm rheostat, 1/2 amp pico (axial) fuse,a dc supply (12vdc works good) and a blue led, and connect them in series. the rheostat will allow you to set the intensity bf the led by varing the current. the 100 ohm resistor is used as a current limiting load.
there are alot of online vendors that sell those led moonlights, if you like to save yourself the hassle of doing it yourself.
pictured is a basic wiring schematic using a battery, a resistor, and led.
It has a calculator that tells you what size resistor to use. You will need to the know the specs of your LED (the forward voltage and current.) I don't know the exact science behind it, but you can not just match the LED's required voltage to a power supply, you need the resistor in-between.