When shaving your armpits, always use a sharp razor, as a dull razor blade increases the risks of getting razor burns and creates irritation. Your skin is sensitive under the arms so a recommendation is to use warm water and soak the skin to get rid of sweat and deodorant and to soften the skin and make the shaving closer. Use shaving cream if needed. Lift your arm above your head and shave gently with short strokes in all directions. When you are done, let the area dry and then moisturise. Avoid using deodorant on newly shaved skin, since the skin needs to rest for a while.