The logbook is a legal contract between you and the RTA. You and your supervisor are declaring that you did complete the required hours, fraud is a serious offense and can incur serious penalties and suspensions from driving.
Always fill the log book in pen, not pencil. Never use liquid paper, show errors with a line through it. Never loose the front cover of the logbook as this is the contract part with your signature and shows ownership of the logbook. Look after your logbook by putting it in a zip lock pouch. Always take digital photos of the pages as you complete them, in case you loose the logbook you will have a record of hours already done.