Project notebooks, like other notebooks, usually don't have a specific format since they are personal documents. However, you should always date your entries and make sure your notebook is well organized. A bound book and page numbering keep your notes in order.
A more specific organization option is to use two columns for each entry. One column becomes the task column, where you document what you are doing and discovering. The other column then becomes the reflection column, where you record your thoughts on what is happening and any questions you have. Using the same organizational form for each entry helps you locate specific information quickly.
You might also consider keeping an electronic notebook (on a disk or hard drive). However, you won't be able to draw sketches as you would on paper and you would have to print your entries before meetings.