You're in luck, because that mix of high/low style is totally in fashion right now, and there are plenty of editorials, ads, fashion shows, celebrities, and streetstyle to get inspiration from.
I would start by focusing on glamourous or ladylike accessories and hair/makeup, and incorporate those into a casual jeans/scruffy style. You could nix the idea that casual has to mean scruffy, and wear very refined casual clothes. Like replace hoodies with blazers, jeans with trousers, t shirts with button-front shirts etc.
Or you could play up the contrast between a "scruffy" outfit paired with, for example, red lipstick and nails, polished hair, and fancy jewellery. Look at the model streetstyle thread - I adore those pictures of models outside of fashion show venues, where they often have super-polished hair and makeup from a previous show, but have changed into their everyday clothes.
For classic inspiration, take a look at Grace Kelly's thread - she always looked glamourous in jeans and a tshirt. Also look for screen shots of The Misfits with Marilyn Monroe.