Develop a research plan
At this point, you have a general idea of what your project will entail. It is now time to put together a more detailed plan on how to complete the project. The plan should address the entire project, including any preperation that is necessary, such as building research apparatus or aquiring specialized research materials. It should present any experiments and how results will be evaluated.
A research plan needs to be detailed. It is not sufficient to say, "I am going to make an airplane" or "I am going to see how a plant grows in dirt." The plan needs to indicate all the steps you plan on taking such that another person reading the research plan could repeat your process.
The research plan must address safety and health issues. This means you must understand the materials and processes involved, as well as the risks and the way to minimize those risks. The risks should be enumerated as well as how they will be addressed. For example, if animals are used in an experiment, then their care, feeding, and housing after the experiment must all be addressed in the research plan.
Your mentor/adult-in-charge/supervisor will review your research plan and may suggest changes. Likewise, the research plan may have to be reviewed by a safety review committee before your experiments can begin. The more details in the plan, the better the chance that it will be approved without changes.
A research plan needs to be as long as necessary. It may fit on one page or many pages.