A template is where you can make your mistakes without wasting your good materials. You can test the size and fit, make alterations freely and mark it up with notes. You then use the template to create a pattern so you can make a good version using your final materials.
Templates can be made with cheap materials like foam, cardboard or paper. You can probably procure a mockup material for cheap if you don’t have these materials around your house already! If you’re working with papercraft/pepakura files, you can print the net out onto printer paper.
You can skip the template and make your helmet with your final material, but mistakes will be more costly and there is less freedom to experiment to find out what works best.