UNO Atmega16U2 Firmwere

I am going to make my own board based on the UNO and need to know where i can down load the software to put on the Atmega16U2 for the USB to work. Is there any instructions anywhere i can get info on this? and how to install it?

Any help on this will be appreciated.

You find it in the directory hardware/arduino/firmwares/ of your IDE installation. There is a Readme.txt that explains how to use it.