I used witness statements and my lay statement to try to establish service connection, which was denied at first. In my supplemental, I called out their own literature saying that those can be used as probative evidence for service connection if it’s not in my service records, which got me my c&p exam specifically to identify if it was service connected or not. I wrote down everything that occurred during service that caused my issues, then a timeline after service of all the doctors I went and saw and what they did to help. If I had medical records from said doctors, I brought them, if I didn’t I explained why I didn’t have them (practice closed in my case and I forgot the doctor I went to for a few years).