First of all there is no need for Name and address in one of these letters. You simply start with ‘Dear Reader’!
Paragraph 1: Explain what the purpose of the letter is, and keep things very relaxed and informal with a couple of introductory sentences.
Paragraph 2: Expand on the first paragraph and explain a bit more about what this letter is directing the reader to. Here you should be using two or three sentences.
Paragraph 3: This should be the conclusion to your letter, and it is in this paragraph that you will be trying to direct the readers to whatever it is you are pushing them towards. Try to include a call to action for your readers and remember to thank them for their time.