Apart from having the collection agency's logo on it, the letter/dunning letter is already fully formatted. All you have to do is fill in the name of the debtor and the amount of the outstanding debt.
A dunning letter in the name of a collection agency usually makes more of an impression than a letter sent in your own name. This will increase the chances of payment and save you the costs of a collection process - which often does not benefit the customer relationship. Dunning letters are available in both Dutch and English.