How do you write your commander a letter?

I have a memorandum for involuntary separation CH 14-12b. The NCO basically sent this all the way up the chain of command and said that he wanted me out. so they started the packet and everything. It says i can write something for my defense but i have no idea where to start or how to address the letter. I cant afford to be kicked out and the allegations against me are 80 percent here say and 20 percent i wasn't told the place of duty and got in trouble when no one answered their phone. I am in MEDAC and the hops ital Commander will be making the final decision. I have a wife and a child and I cant get kicked out right now. I am trying to write something in my defense but I don't know how. can anyone help me with the format and how its suppose to look and our sound?