Stand in front of a basin. Mineral spirits are highly flammable and must not be used near sources of heat.
Pour a liberal amount of mineral spirits over your hands. Rub your hands together vigorously, until you can feel the urethane begin to lift.
Rinse your hands with warm water. If there is still residue on your hands, use more mineral spirits, being sure to rinse afterwards. Repeat until all the urethane has gone.
Wash your hands with hand wash. This will remove all traces of the paint thinner.
Dry your hands with a towel. The mineral spirits will dry your skin, so moisturize your hands with lotion.