First, let's assume that you are a potential plaintiff. You have a claim or several claims against someone else and you know how much you will sue for. You may just resolve this thing without going any further. There's no harm in asking. Asking for what is owed you can take many forms.
Some people have a talent for writing demand letters with increasing degrees of seriousness.
It is a simple matter to put your demand in writing. Deciding exactly what to write will help you be clear about what you want in order to resolve your dispute. Your demand letter can also guide you in defining the issues that may result in a trial if you need to sue. If you get a demand in writing, send your response the same way. But be sure that you respond.