Using LEDs from DC is fairly simple.
Say you want to run the LED at 20mA
R = (E - Vf) x 1000/1
Where R is the resistance in Ohms
E is the DC supply voltage
I is the LED current in mA
Vf is the forward voltage drop
of the LED (typically 2V).
Here are resistor values for sample voltages
6V R=220 Ohms
9V R=330 Ohms
12V R=560 Ohms
24V R=1200 Ohms
