External peripherals emulating keyboard input

I am trying to develop a device which would attach to my Droid and send keystrokes to it, but I need help understanding how the USB OTG works. I have the basic pinout, but is there a reference and instruction set guide anywhere, preferably with example code? Has anyone had any luck developing similar peripherals?

http://wiki.droid-devs.com/images/7/7b/Otg.png

Thank you for your time.