They do it themselves - they are grown women who were probably doing their own hair and make up for years before they even got hired as as cabin crew. They might visit a hairdresser once in a while, but otherwise, they do their own hair - to company standards, of course.
no they have to know how to do it themselves. If you're hoping to be a flight attendant, i would practice putting your hair up in a bun and doing your make up.