Think about how you will document your experiment as you carry it out. Your observations are data. What tools will you use to make your observations? Will you write notes, draw pictures, take photographs? What tools will you use for measuring changes? Scales, rulers, micrometers? Everything needs to measured carefully and described specifically when you document your data.
If you need to change something about your procedure or one of your materials, document this, too. Later on you will be able to explain how this change might affect the outcome of your experiment when you analyze your data.
Tip: The independent variable will have an effect on your dependent variable. Pay attention to this effect and measure it carefully. This is the data that you will want to analyze later.