If you have a serial ELM327 interface, I think you just have to plug it to the serial plug of an Arduino clone. I don't know if you need a straight cable or a cross one however!
If you have a USB version you will have to wire pin 16 of the ELM327 to pin 1 (TX) of the Arduino and pin 17 of the ELM327 to pin 0 (RX) of the Arduino.
Project is described here
OBDuino - opengauge - OBDuino Description - Google Code