A resistor is the easiest way, check out this online calculator.
**broken link removed**
Keep in mind you can't regulate the current if the resistance is too low, for good measure pick a number of LED's that will result in a resistor around 100ohms.
I put in 4 LEDs with 2.4V voltage drop at 20ma with an input of 12 volts = 120ohm resistors for a 4 led series. .048 watts dissipated in the resistor. Watch that disipation value carefully because high disipation resistors are two things, expensive and wasteful. Common cheap carbon resistors don't go much bellow 1/8 of a watt. I would recommend using thin film resistors, not as cheap but much more stable.