Examples: demanding payment on a debt – fairly simple letter; Requesting that an employee be re-hired – more complex.
If the issue goes to court, the letter may be shown to the judge or jury. It is good practice to send your client a copy of any document you generate on their behalf.
A demand presumes a right, an absoluteness, a certainty about what the letter is requesting, and the tone should be reflective of that.
A demand letter with a bullying tone is often unpersuasive, and will have a negative effect on a judge or jury.
Simply express a professional view of how the law treats the facts. Your message is that the other side did something wrong, but it does not have to be that they are bad people. If you insult your recipient they are much less likely to give you what you want.
Consider the relationship between your client and the recipient, for example if the dispute involves a contractual relationship that will not terminate for several years. Consider your client's goal in the relationship and whether they need to avoid damaging it.
You should provide enough information so that a reader can gain an overall understanding. Readers tend pay closer attention to opening and closing sentences -- place your favorable facts at the beginning and end of the factual statement. Use short sentences -- they draw the attention of the reader and are easy to remember, making them more powerful. De-emphasize or neutralize unfavorable facts, mention them only once or as few times as possible. Place unfavorable facts in the middle of the section. Use long sentences -- longer sentences tend to downplay the impact of each fact.
Threats anger people and make them want to fight back. Threats can also offend 3rd parties like judges and juries.
If the explanation is extensive, a summary of the explanation
There are Federal Evidence Rules that pertain to the admissibility of demand letters.