The original idea comes from Laurier's Handy Dandy Little Circuits.
Step 1: Step 1: Get the components!
1 Resistor (try different values, in this tutorial I used a 510 Ohm resistor)
1 LED (Pretty much any single color, normal 20mA LED.)
1 Capacitor (Yet again, try different values, must widthstand atleast 16V. In this tutorial I used a 470µF one.)
1 2N2222 transistor (I have not tested the 2N2222A type, so not guaranteed to work, but feel free to experiment with different types.)
And a variable powersupply! (So you can vary the voltage you power the circuit with.)