Setting Flash Channel; To change the flash Channel, press the button underneath CH on the LCD(top arrow). You can either cycle through the channels by continuously pressing the button, or by highlighting the channel by pressing the Gh button once, then use the command dial(lower arrow) to select the channel.
The channel of the remote flash must be in the same channel as the master unit, or the remote flash will of course not fire.
Done your SB 900 is now set up as a master wireless flash.