Name of recipient
Dear (insert last name here)
Here you should state the purpose of writing the letter, and explain that you are making a declaration. This should not be more than two sentences.
In this second paragraph you should go on to make your declaration and explain the reasons behind it. This can be two to three sentences, but ideally not more than that.
This section is should be a concluding paragraph which should reiterate your statement and if you wish, it should also have some sort of call to action for the recipient of the letter in terms of your declaration. You should also finish with a polite sentence thanking the reader for their time, and if appropriate to the letter, indicate that you are looking forward to hearing from them soon.