It will take more than a letter I promise you. When I DA'd myself several years ago I had to attend about 6 to 7 months of meetings without missing to many before they would even consider reinstating me. It was all I ever knew for almost 30 years and now after being back for almost 4 I realized I made a mistake going back. I originally came back to have a relationship with my family again. Now that I have found so much conflict based on false prophicies, unscriptural practices, etc. my wife and I have stopped going. If you are doing it for family please be careful because if you are not practicing that religion whole souled.....they will call you out on it every chance they get. Every meeting you miss, every hour of field service you don't participate in, every comment you don't answer.....just wait and see. I basically told my family this last week that we were not attending the meetings any longer. I can't do it to my kids nor my wife anymore. I told them I loved them and we will not be DA or DF'd because we haven't done anything, but we are here to love like we always have, but we will not have the same beliefs. We will see how it goes. As for your letter, there is probably no need for one until you have been going back for 6 months or so.
Best wishes!