Choose a project that you think will be interesting to learn about and complete. When choosing your project, be sure to pick something you will be able to finish satisfactorily in the amount of time you have available, and be sure that whatever you pick is within your abilities to build. If you want to build a miniature solar-powered car, for instance, take into account the skill level needed to build it, as well as whether you can get hold of the materials you will need.
Plan your project. Conduct some research on the best way to construct your project. You will probably need to complete a report or poster board with your project, so write down the process you plan to follow and how you expect your completed project to work before you start construction. For example, if you plan to build a model roller coaster, write down what you plan to make it out of and how well you think the completed model will work.
Identify and gather the materials you'll need to construct your project. Mechanical science fair projects often require an array of materials. Before you start building, make sure you have access to everything you'll need to complete the project.
Begin construction. Follow the plan you outlined earlier. Reference a guidebook or Internet Web site for instructions, if necessary. If your project will take more than one day to complete, decide how much to do each day. For complicated projects, and those that require the use of potentially dangerous materials, ask your parents or teacher to supervise you while you work.
Test your completed project. Does it work the way you hypothesized it would? What could you have done differently? Write down your observations and write up your report or poster board.