Many have been attending dance, acting, singing, and music classes since tiny, but the type of classes where children are the client and really encouraged and expected to shine and have self belief.
Sport and language classes are also often provided, but they are individually coached.
They are automatically taken to orthodontists, hairdressers, and brought reasonable quality clothes and good shoes, so don't worry quite as much about their appearances when really young.
They are taught how to eat at table, sit, stand and walk well, how to make conversation with adults, and travel broadens their minds before they know they need it it. They have some idea of cultural references, and issues with education are generally caught quickly and tutoring provided so they don't 'fall behind.'
Quality things and places are normal and open to them, security don't magically appear if they enter Fortnum and Masons or expensive shops, and they aren't placed by the toilets at the back in any good restaurant, so they feel comfortable and entitled to be there, or to be waited upon, served etc.
They learn from a combination of classes, parents, the reactions of those who services are purchase from, and osmosis how to present themselves to the world as belonging and being entitled to be wherever they choose to be and don't generally suffer from inferiority complexes. (as long as they don't go slumming at which point they often stand out as potential victims)
Class comes into it if and when it restricts movent and money, and a sense of 'entitlement' (as in towards anything better than what one has) and when other parts of society ensure their views of you are known, and when your own part reacts badly to your choices, and schools attempt to ensure you know your limitations better and long before your potential's been thought about,
I've been torn apart for my views on how to not let class and difficult beginnings hold your Dc's back before, so tin hat on.
Imagination, vocabulary, appreciation of quality, valuing education of all forms, manners, decency, a good sense of self, skills, and in our case the simple decision that whatever has been tried and tested to be good enough for the comfortably off, is likely to be good enough for my family too, even if I have to find different ways of obtaining it, and add it to our other skills.
We're from LC (me) WC (Dc's) with an appreciation of all sorts and all able to hold our own in any decent company of any class, and my Dc's and DGc's can and are expected to code switch and move between it all entirely comfortably and treat others with equal compassion, and remember the weight of a gilded cross can be heavier than an wooden one.