If you have lost your RC book, then you have to register a complaint with the police at the nearest police station. You must provide all the details about how and where exactly you lost your RC card. Then fill in the required forms and mention the registration number of your vehicle, model, your name and address, the chassis number of the vehicle, engine number, etc.
Sometimes there can be a substantial time frame between the purchase of your car and obtaining your Registration Certificate (RC), then within this period, car insurance claims can be done without an RC book and with other requested documents while filing the claim.