If you have a costume that requires a helmet, you can make one out of corrugated cardboard. The exact method of making a cardboard helmet will vary depending on the type of helmet you are making--whether it's a knight, gladiator, spaceman or other helmet. While the following provides some general tips, it helps to look up any procedures for making your specific type of helmet.
Design a template for the helmet, measuring your own head's dimensions. You should draw out the template to be at least an inch larger than your own head's measurements. You'll also need to draw it out in the correct design if you are creating a specific type of helmet, such as that of Roman or Spartan warriors, Iron Man or "Star Wars" stormtroopers.
Cut the pieces for the helmet out of the cardboard. While it can vary depending on the style, you will likely need at least one piece for the sides and possibly four or more wide strips for the top. Cut two or three circles that measure the circumference of your own forehead, plus an extra inch.
Wrap the side piece(s) around one of the circles; the place where the ends point toward each other will be the front of the helmet.
Cut another of the circles in half and one of those halves in half to make two fourths. Glue these pieces together so they appear to form a cross from above and a dome from the side. Glue this assembly on top of the helmet sides. You can then cut away the extra material from this assembly so it looks like two arches that connect in the middle.
Place the top pieces over the cross-dome assembly on the top and glue them to the sides, trimming them where necessary. There are several ways you can do this, depending on the style of helmet you are making. If using strips, you can wrap a couple of them across the top of the dome and then cut out pieces to glue onto the "wedge" holes in the top. You can also place one of the circles on top and fold it down over the dome.
Decorate the outside of the helmet as you see fit. This can involve drawing and painting it the colours you want, or wrapping aluminium foil around it.