A demand letter sets out the claim, the basis for it and what you want. Its real function is to move the matter to a conversation without the cost of a complaint.
Tone matters more than people expect. A letter that overstates the claim invites a dismissive reply; one that sets out the facts precisely and states a specific remedy is harder to ignore and reads as preparation rather than posturing.
It also fixes your position in writing at a point when the other side's response is still uncoached, which is frequently useful later.