Pay attention to everything around you while completing the observation. If you focus too much on just one aspect of what you're observing, you're likely to miss other things. For example, if you're observing a teacher, also pay attention to the reactions of the students.
Take detailed notes and use shorthand when possible. The more notes you take about what you're seeing and experiencing, the more data you'll have to work with when you write up the one page summary.
Write your notes objectively. Objective data tells you more than subjective data when you review your notes to write the summary. For example, if you see a student yawn while the teaching is lecturing, do not write "Student lays head on desk from boredom." Instead, write "Student lays head on desk." Later in your observation, you might note the same student blowing his nose and coughing, meaning the student has a cold and that could be why he laid his head down, which has nothing to do with the teacher's lecture.
Pull what is meaningful for your purposes from your notes. Exclude other observations you had. For example, if you're observing a teacher to determine the adequacy of her job performance, the fact that a student with a cold laid his head down should not be included in your write-up. However, if the majority of the class did not seem to grasp a concept she explained, this could have something to do with her job performance. By using only relevant data from your notes, you can keep your observation summary to a one page length.
Draw conclusions. The last few sentences of the write-up should include what you ultimately concluded from your observation. Continuing to use the example provided in step 4, you may write your inference that the teacher is performing her job adequately.