Recall the events of the day's dive immediately after you've returned back to the shore or boat. Once you've dried off and you're comfortable, open your binder or software program and fill in your name, date, and dive location.
Continue filling in the necessary information about the dive, which includes what type of exposure suit you wore (skin suit, wet suit, etc.) and the millimeter of protection. The following information you enter is of utmost importance: fill out the maximum depth you reached during the dive, the beginning time of the dive before immediate submersion (SI), your pressure group, or PG (refers to amount of nitrogen buildup in your body), time on the bottom, and your PG upon resurface, and time spent at the surface.
Fill out the remaining information, including weather and water conditions and record any notes of the day (how you were feeling, aquatic life spotted, etc.). This information is valuable when planning your next dive.
Plan your next dive using your log book; use the information you entered to determine the safe amount of time you must spend at the surface before you dive again. This can be determined by your dive planner. If there will be significant time between your dives, use your log book as proof to the dive master or instructor of the number of dives you've done and to what depths.