Not Guilty!!! I Really Didn't See That Sign!!

I was accused of disobeying a traffic signal (No Turn Here) in NYC. I was just visiting that day first off, and I honestly did not see that sign. I would never disobey a traffic sign... especially not in NYC, where cops are all over the place. I pleaded not guily and have been given the option to write a letter to the judge. What do I say? How do I say it? Can anyone tell me where I can see a sample letter like this?

PLEASE HELP!!!

Just 'cause you ain't seen no sign don't mean you ain't guilty, cause you are