Mainly just put down what is pertinent to you and your technical interests. For instance, you might want to time your flights with a particular type of fuel in a particular engine and see if tuning it for weather factors will effect its fuel economy. Or you may change your TX setup and log the changes in flight characteristics of a particular plane. Or you may try different props,glowplugs, landing gear,spinner, whatever and see if a particular plane/engine combo works for you better. I always have a page for just notes, things learned both in flying and in interacting with fellow pilots. I use FlightLog too. I was putting it in a notebook but just got a Computer(notebook) and can use it now.