For the purpose of this example, we'll choose a simple, small-scale experiment. In our experiment, we'll test the effects of different aerosol fuels on the firing range of a potato gun. In this case, the type of aerosol fuel we use is the independent variable (the variable we change), while the range of the projectile is the dependent variable. Things to consider for this experiment - is there a way to ensure each potato projectile has the same weight? Is there a way to administer the same amount of aerosol fuel for each firing? Both of these can potentially affect the range of the gun. Weigh each projectile beforehand and fuel each shot with the same amount of aerosol spray. If we're testing hair spray, cooking spray, and spray paint, let's say that the hair spray has an aerosol propellant with a higher amount of butane than the other sprays. Because we know that butane is flammable, we can hypothesize that the hair spray will produce a greater propulsive force when ignited, sending a potato projectile farther. We would write our hypothesis as: "The higher butane content of the aerosol propellant in the hairspray will, on average, produce a longer range when firing a potato projectile weighing between 250-300 grams." In our experiment, we'll test each aerosol fuel 10 times and average the results. We'll also test an aerosol fuel which contains no butane as our experimental control. To prepare, we'll assemble our potato cannon, test it to ensure it works, buy our aerosol sprays and carve and weigh our potato projectiles. Let's also create our data table beforehand. We'll have five vertical columns: The furthest-left column will be labeled "Trial #." The cells in this column will simply contain the numbers 1-10, signifying each firing attempt. The following four columns will be labeled with the names of the aerosol sprays we're using in our experiment. The ten cells beneath each column header will contain the range (in meters) of each firing attempt. Below the four columns for each fuel, leave a space to write the average value of the ranges. We will use each aerosol spray to fire ten projectiles, using the same amount of aerosol spray to fire each projectile. After each firing, we will use a long tape measure to measure the range our projectile traveled. Record this data in the data table. Like many experiments, our experiment has certain safety concerns we need to observe. The aerosol fuels we're using are flammable - we should be sure to close the potato gun's firing cap securely and to wear heavy gloves while igniting the fuel. To avoid accidental injuries from the projectiles, we should also make sure that we (and any observers) are standing to the side of the gun as it fires - not in front of it or behind it. Let's say we found that, on average, the hair spray shot the potatoes the farthest, but the cooking spray was more consistent. We can represent this data visually. A good way to represent the average range for each spray is with a bar graph, while a scatter plot or box plot is a good way to show the variation in each fuel's firing ranges. Reflect on your experimental results and provide any supporting statistics. Based on our data, we can say with confidence that our hypothesis was correct. We can also say that we discovered something we didn't predict - that the cooking spray produced the most consistent results. We can report any problems or snafus we encountered - let's say that the paint from the spray paint built up inside the potato cannon's firing chamber, making repeated firing difficult. Finally, we can recommend areas for further research - for instance, maybe with greater amounts of fuel, we can achieve greater range. We can even share our results with the world in the form of a scientific paper - given the subject matter of our experiment, it may be more appropriate to present this information in the form of a tri-fold science fair display.
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One-sentence summary -- Pick a topic and define your variables. Make a hypothesis. Organize your data collection beforehand. Conduct the experiment. Analyze the data. Make your conclusions.

Seeds can be ordered online or from gardening supply stores and nurseries. Popular varieties of watercress include English Watercress and Broad Leaf Cress. You can also start growing from mature watercress purchased at a supermarket or farmer's market. Cut the ends, then soak the base of the stalks in water for a few days to encourage root growth and proceed to plant them in soil as you would from seed. Choose a large container or planter with drainage holes that is at least 6 inches (15.2 cm) deep. Add a layer of landscaper's cloth at the bottom of the container to keep the potting mix from escaping when you water. Add pieces of broken pots or small pebbles to the bottom layer of the container to allow for good drainage. You can also use multiple small containers and place them in a larger drainage tray. Plastic containers are recommended over terra cotta ones, which can dry out too quickly for watercress. You need to water the plant often. It's soil should be wet at all times. You can put excess water in the tray or bucket to keep the plant wet. You can also place small pebbles in the drainage tray to allow water to flow freely into the growing container. Use a soilless mixture that drains well and contains peat moss and perlite or vermiculite. Leave approximately two inches (5 cm) of space to the top rim of the container and water the mix well. The ideal pH of the potting mix should be 6.5 and 7.5. Place the seeds 1/4 inch (.64 cm) deep in the potting mix, allowing three to four inches (7.6 to 10.2 cm) between each seed. Soak the potting mix deeply enough so that water fills the drainage tray below roughly halfway full, but doesn't rise higher than the growing container. Replace the water in the drainage tray with fresh water every two to three days. Make sure the tray never dies out. Check it daily to see if you need to add more water. To keep the soil thoroughly cover the surface with a thin, clear plastic sheet that has small holes poked in it, which will retain water and allow airflow. The sheet can be removed when the sprouts begin to appear above the soil. Mist the soil surface well with water in a spray bottle every other day. Position the watercress where it will receive roughly six hours of natural light each day, but try to avoid harsh, direct rays that can burn the young plants. You can keep the containers indoors or when the weather is regularly between 55F and 75F (13 and 24C) where you live, you can place the container outside during warmer months. Add a small amount of water-soluble, all-purpose gardening fertilizer to the water in the drainage tray at the package-recommended rate. Once the plants have grown roughly five to six inches (12.7 to 15.2 cm) in height use kitchen or gardening scissors to trim the top four inches (10.1 cm) of the plant as needed. Avoid taking more than a third of any plant when cutting to allow the plants enough foliage to continue growing. Periodic harvesting helps encourage new growth. Rinse the watercress in cool water, dry it and use immediately or wrap it in bundles and store in the refrigerator for use with a few days.
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One-sentence summary --
Purchase the watercress seeds. Prepare the container for planting. Place a larger drainage tray or bucket beneath the planting container. Fill the growing container with potting mix. Sow the watercress seeds. Water heavily. Place the container in indirect sunlight. Fertilize the watercress. Harvest the watercress. Wash the watercress.