For me it almost always starts with a nice retro inspired dress of skirt. Once quick way to make it modern is to do the hair quite simply and pair contrasting prints (very on trend for Fall and Spring).
yellow is a 1949-summer-ensemble-by-jacques
A nice wide brown belt (less formal then black) or flat slouchy boots are good casual accessories.
One of my very favorite things to do is pair modern patterned tights with a retro dress. Also mixing black and brown instantly makes something slightly more casual and generally less retro.
Another thing to do is take cues from looks worn by less wealthy ladies of the past. Fewer accessories, simpler and more comfortable dress.
Bomb Girls
Here are a couple of retro yes modern casual looks . . .
Messy hair is definitely not retro, socks with clunky shoes, a causal cardy or blazer instead of something tailored thrown on unbuttoned over the top.
Pairing a very simple element (like a plain jersey top, or over-sized cardy) with something extravagant is also an option for playing down the super vintage elements.
One of my very favorite things to do is to take a very sweet romantic dress and toughening it up for day (or night) with leather, a motorcycle jackets or doc style boots.
Here are a few of my favorite casual retro daytime looks incorporating some of the dresses and skirts you have seen me wear at more formal occasions.
