Choose a subject for an experiment. This can be anything you can observe and measure in the natural world or in a social science situation. For example, you might consider how the amount of water a certain plant gets affects its growth.
Determine the end result that you are testing. This is the dependent variable. Be sure it is something that you can accurately measure. In the example given, you would be testing the rate of growth of a plant, measurable by its height over time.
Choose a variable that will specifically affect the dependent variable. This is the independent variable because it is changed by the scientist and does not depend on another variable in the system. Again, be sure this is a measurable value. In the example, the independent variable would be the amount of water the plant receives.
Use existing knowledge to make an educated guess as to how the independent variable will affect the dependent variable. In the example, you know that plants need water to grow, so you might guess that plants that receive more water will grow taller than those receiving less water.
Phrase your educated guess as an "if, then" statement. This is the format of a formal hypothesis. In the example, your hypothesis could be: "If plants need water to grow, then plants that receive more water will grow taller than those receiving less water."