Put all the records in order:
First of all you will have to collect all the evidences and preserve them. You better make a file and put every relevant document in order. Evaluate your damages and find out whose fault is it. You should collect all the bills, receipts or any other proof to make your case strong.
You may take pictures or make video of the evidence as it will help you to prove the other party guilty. Try to think with the mind of a jury and try to cover all the aspects. Do not feel hesitation in recording anything for example if you are a victim of car accident then you should have clear pictures of the damaged car, broken pavement etc.
Do extensive research:
Before filing your complaint or submitting demand letter, you should try to find out which law your opponent violated as it will be helpful to prove them guilty. Mostly people think that it is very difficult but you may take help from many online law libraries to know the valid law.
Obtain copies of important documents:
You should get copies of documents such as police or medical report as these documents will strengthen your case and provide the jury an official view of investigating authorities.
Now, when you have gathered all the data, start writing your letter. You should be direct, clear and to the point. Do not play with words rather describe complete story in simple words. Use pictures, mention damages, compensation and relevant laws to support your claim.
Once you are done with the writing, attach all the documents and dispatch this letter through a certified service.