Basics: Entries should include a descriptive title for the experiment (and continued from information if continuing from another page) and date of the entry.
Purpose: State the purpose or aim of the experiment. Why are you doing this experiment? What do you hope to find out? Are you testing a specific hypothesis? Trying figure out the right concentration of salt to use in an assay? Exploring the behavior of your system? Confirming a previous result? Purpose statements can be (and usually are) brief.
Methods: This is where you keep detailed notes on what you actually did for the experiment. The notes should be detailed enough for another researcher to come along and replicate your exact experiment without having to consult you. Any potentially important information should be kept – such as if a plate was accidentally left open for an extended time you would note that so if contamination is observed later you can connect that back to the mistake.
Results: All raw data and potentially relevant observations (note the use of ‘potential’ here – you often do not know the importance of an observation at the time you make it) should be clearly organized and recorded in this section. You should use notation that is complete. So if you record colony counts for plates 1-12 make sure that the entry also records what was plated on plates 1-12. Analyses are usually included here (such as average calculations, p-values, or graphical representations of the data) but are not absolutely necessary so long as the raw data is recorded. Graphs can be glued or taped into notebooks but original data should be recorded on the notebook page in pen so that it can not be argued that the data was later changed.
Conclusions: Any conclusions that you draw from an experiment should be noted in your notebook so that you can follow your experimental process. Conclusions might address your original purpose (for example, this experiment supports our hypothesis) or may address technical issues (for example, since so much contamination was observed we believe our yeast stock might be contaminated) or unforeseen results (for example, although unexpected we saw that yeast seem to grow better inside paper bags than in incubators).