Having spent Lockdown 2.0 watching old Audrey Hepburn films, I have concluded that to look classy you have to have small boobs, a long neck, and smooth skin and hair (no curls). (So I'm already ruled out, and can do whatever I like.)
A wardrobe of clothes tailored to your body (which of course never changes shape) would be ideal. But failing that, you must wear wool coats, long or short but not anoraks or puffers. They may be colourful, but never pastel. And leather shoes, not trainers, Converse (*except possibly with bare ankles, in summer, on holiday).
It would probably help if you lived between about 1940-1965. And wore gloves.
But (sorry, Grace Kelley) do not go too blonde. In fact, my final observation is that what looks good in photographs or on film (sharp contrasts of makeup and colours, including the supposedly classic red lipstick and fingernails, or eyebrows and lashes much darker than hair, or contouring) never looks good in real life.
In short, we are all doomed by unfortunate genetics, living in the wrong era, and the general impossibility of reincarnating Audrey Hepburn. It is some consolation to me that even she didn't look that good in Sabrina after her makeover -- too stiff. But having Humphrey Bogart on her arm did help the look.