I just went through this last month. A couple things to understand. First the insurance company will press you to make an immediate decision on their offer. They will claim to only provide rental coverage for three days post offer. Do not accept the initial offer. Ask them to send you the vehicles they used to estimate the value of your loss. You then need to find flaws(year, features, higher mileage) in these cars.
Do your research and find comparable cars(year, features, mileage), through auto trader, kijijji, car dealers etc. You will need 8-10 comparable vehicles. Next you need to send the insurance company your counter offer, and comps. They will drag their feet responding to you, as they are trying to press you to accept their offer. Keep pushing through rep, and supervisor for a better offer. If they do not agree, escalate to the insurance companies internal ombudsman. They can negotiate further.
My insurance company initially offered me $7,600 for my totaled car, I ended up settling at $10,450.