Determine what it is you need to make. You will want to know the size, shape and look of your final creation. This way you will be able to gather the right amount of metal needed. At first you may want to include a healthy amount of waste metal, just in case you don't get it right the first time.
Break the final product up into smaller pieces. Unless creating a very simple object, you will almost certainly need to make manageable-shaped sections to be riveted or welded together later. Be sure that your pieces are all at the same scale. Measurement is key.
Draw out your plans on paper. Your individual pieces will each constitute an individual drawing. In many schools, for the first few projects, students can use paper to make a full three-dimensional model, just to get the feel for it.
Transfer the plans to the metal sheets. Be sure that the lines are exactly what you want, especially for any curved or rounded areas. This will be vital when creating an arc in the manufacturing steps. If your lines are wrong in the layout then your final product will not come out correctly.