Decide on something to make. To do this, consider something that fulfills a purpose and is made from multiple parts that can be preassembled or assembled on site. This object needs to be manufacturable (or be able to have the parts built in a factory for easy later assembly). Examples of prefab objects include prebuilt sheds or modular furniture that you assemble at home.
Draw the concept art of your object. This will be your initial design and it may be altered over time. Keep in mind that your design needs to be simple, easy to manufacture and easy to put together.
Translate your design into measurements. As a general rule, assume that the weaker your material (some woods, foam and cardboard), the more space it will occupy while strong materials like steal or composite metals can be used for extremely light, delicate fine work that is strong.
Build a functioning prototype of your initial design in your initial material. Use this prototype to test the object. For homes, you should test the time it takes to assemble, how well the utility systems work, whether or not the materials used can stand up to the environment and more. For example, a chair should be tested for the weight it can hold and how easily it can be assembled. Use this testing period to gain feedback on your design, such as whether people find it attractive, its usefulness, how easy it is to use or if it does not fulfill expectations. Write down what aspects of the design need improvements and what aspects work well.
Alter your prototype according to your research. Pay particular attention to safety. When it comes to prefab it is best to build to size, even if it is a large prefab home. This will help you verify your measurements and ensure you have chosen the best materials. After your design is perfect you can begin the manufacturing process.