Describe where documents, computer files and other proof are located.

List names of all the others who have knowledge of the matter. Classify them as friendly or not, still employed or not.

Identify supervisors and others who likely would have participated in wrongdoing.

Describe the complete corporate structure of the offending company.

Make a list of the specific laws, regulations, rules and procedures that you believe were violated.

Writing too much is better than not writing enough. Don't exaggerate. It's never wrong to say you don't know something. Don't hold back the facts that might harm the case, or any negatives in your own background we need to know everything. Your success depends on it.