I found the main idea for this from Family Fun magazine but there was little instruction. I like pictures - so I took some pictures in process of making my son a viking helmet.
1. Get a hair head band - a plastic or sturdy one - and shape tin foil horns onto the head band.
2. Wrap the tin foil horns with masking tape. You can paint these white if you want but the tape looks fine without it.
3. Take a old baseball cap and cut the bill off. Place the head band with horns on top of the hat.
4. Start taping the head band onto the hat with silver duct tape. Make sure the hat stays the right size by placing it on your child's head or finding something like a ball to put the hat on.
5. The hat can be decorated with electrical tape of different colors or leave it plain. Make a shield out of cardboard painted a color and then lined with silver duct tape. Tadah! Your viking is ready to sail the seas.