Establish facts. By writing down all relevant facts in the order they happened, you let people unfamiliar with the situation understand what happened. Include dates if you know them. Do not include irrelevant facts in your demand letter because they look good.
Refer to evidence. If you know any evidence (like a contract), you should mention it. While you do not need to attach a copy of the evidence in your demand letter, you can include a copy if it is available. Highlight the pertinent part(s) you are referring to in the demand letter.
Make a demand. Clearly state your demand. How much do you want them to pay you? What specific actions do you want them to take or cease to take? You can create an itemized list to support your demand if you have multiple demands.
Set a deadline and establish a method of payment. Give the other party a reasonable and specific date to respond to your demand. For example, giving the other side at least seven business days to respond would be reasonable. Say when and how you expect to be paid if there is money involved (such as a payment plan). Do you want a check or just cash?
Offer a consequence. In the closing of your letter, let the recipient know that you will pursue legal remedies if they do not meet the demands in your letter.
Note: Never threaten the other party with illegal activity or physical, mental, or other harm. You will be in danger of prosecution if you make threats.