How very sweet of you to help your sister out with her science fair project! The Scientific Method involves 6 basic steps:
1. Come up with a problem you are trying to solve (usually in the form of a research question).
2. Form a hypothesis, or educated guess, about what will happen in the project or experiment.
3. List all supplies or materials you need.
4. Write out a detailed procedure.
5. Observe and collect data.
6. Come to a conclusion - what is the answer? Was your hypothesis correct?
Below, I'm including a couple articles about the Scientific Method and getting started with a science fair project. You may find other helpful, more detailed information in these articles.