To use these beanies, simply put them on your head. They're one size fits all, so they'll be comfortable for most people. When it comes to maintenance, it's best to follow the care instructions on the label. Usually, a gentle machine wash in cold water is a good option. As for the warning, be aware that the product contains chemicals like cadmium, lead, and nickel. Avoid prolonged contact with skin and keep it away from children as much as possible. If you have any concerns, visit www.P65Warnings.ca.gov for more details.