Pictured is an Arduino Uno board with DMX 'shield' from SKPang.co.uk
Once you’ve downloaded the Arduino software and got it talking to the Arduino board you’ll need to download the DMXsimple library from here and put it in the Arduino Libraries folder.
Then find the DMXSimple.cpp file and change the line #include "wiring.h" to #include "Arduino.h"
The pictures below are of a studio with a single tape of RGB LEDs around the top of the room which make a visible difference even in daylight.
This example code was used in the studio to make the lights; red when the microphone was live, green when not, turn them off (by setting red, green and blue to zero) when a 'lights off' button is pressed, flash green/blue when the phone is ringing and flash green/yellow when the door bell is pressed. The four inputs - mic live, door, phone and lights off are all simply closing contacts that short 0V to pins A2, A3, A4 and A5.
In the Arduino code pinMode(A5, INPUT_PULLUP) is quite useful but you’ll need Arduino version 1.0.1 or later to use it.