Airsoft is a hobby where you use imitated firearms in war games. Similar to paintball, airsoft guns shoot small pellets that will hurt enough to let players know they've been hit, but usually not enough to cause actual injury beyond a bruise. However, players who want to protect their heads can make up a helmet for their airsoft games. This is fairly simple, as long as you have something to use as a base.
Mold your foam over a tight-fitting hat, such as a baseball cap. It should fit well on your head; remove the brim from the cap. Once you have moulded the foam, which should only be a few millimetres thick, to your hat so that it's comfortable. Attach it to the hat using hot glue or nail glue. Don't use too much, because the glue will seep through the hat material.
Attach a thin, plastic covering over your foam. If the hat is the base webbing, and the foam is the comfort padding, then the outer shell is what gives your helmet its look. Plastic fencing, the thin, flexible kind, will bend to fit around the foam padding and it will give your helmet a jungle-esque, Vietnam look. Bend black plastic that's used in cheap dustbins and glue it for a sleeker, Swat-team look. Also, if you want to spend the extra money, purchase friendly plastic, a material that you can heat up and mould with your hands like putty before it hardens into a plastic shell. Get creative with your covering to achieve the look you want.
Test the helmet. Set your helmet on a dummy head (anything round and supporting will do) and fire at it with your airsoft gun. If the helmet and padding stops the round from going through, and the model beneath is unharmed, you have a working helmet.
If you have a helmet you want to cannibalise, such as a bike or skating helmet, all you need to do is modify the cover that already exists to create a functional airsoft helmet.