Writing a good letter of demand involves professionalism in dealing with people and information. There's a method that can be illustrated with a fee for service contract that has gone wrong.
Assume things have not gone to plan under that contract. The service was delivered but the fee was not paid. The unpaid service provider has made repeated requests. Harsh words have been spoken and tension has arisen.
The two parties should avoid the slippery slope. They should try to work together to overcome issues. They've tried negotiation with no resolution. There's been haggling and even verbal abuse. A lawyer might have been called in earlier, but as usually, hasn't been.
The unpaid party now meets with a lawyer.
Having gone to a lawyer, the unpaid party must communicate a story, collect and supply the evidence, and then help the lawyer examine the facts and evidence. If all this is not done the lawyer should not be forming an opinion and giving advice on what to do next.