If you have to write a letter but really don't know how you can do it, what you should write or even where you should send it, you can contact a « écrivain public ».
This person, a public writer is a professional who can help you and write this letter for you.
If you already wrote your letter but don't feel confident, a « écrivain public » can correct it.
Where can I find one ?
Just check the « pages jaunes », search « écrivain public ».
If you're in contact with a social worker you can also ask him/her, they often work with « écrivains
You can also go to your mairie and ask them.
You'll have to pay for this service but it might be very useful and once you have your letter, it makes a good model for the next time.