Talk with the person or group of people you're observing before the observation takes place. Ask them what you should be paying attention to during the observation.
Make your observation for 50 minutes to an hour. This is the timeframe you need to gather enough information to evaluate the observation.
Write down what you're observing. Effective note taking will make the evaluation easier after the observation. Pay attention to what you're looking at and why it's happening. Even if you're not quite sure, write it down anyway. You'll have time to think it through later.
Pay attention to what's going on around the person or activity you're observing. For example, when observing a medical operation, you might be able to learn something from what the nurses and technicians are doing as well as from what the surgeon is doing.
Talk to the person or people you were observing after the observation. Go through your notes with them. Ask questions if you have any and get more information.
Analyze the data you have collected. How you analyze it will depend on the purpose of your observation. For example, when observing classroom instructors, you might be trying to determine which teaching methods are most effective. In this case, use your notes and what you observed to determine both the instructor's teaching methods and whether or not they seemed effective, which would require observing the students, as well. For instance, you notes should indicate how many students were on task and how many were off task. Also, your notes should document the interaction between the students and the instructor as well as the students reaction to what was being taught.