Hailey Bieber's off-duty dressing is unusually easy to reverse engineer, because it is the same three-part structure repeated in different fabrics. There is a slim satin or silk base, usually a slip dress or a satin camisole. There is one oversized layer over it, almost always a blazer or a soft leather jacket, worn open and sized up. And there is a small hard-edged accessory doing the finishing, typically a compact shoulder bag and a pointed or square-toe shoe.

What makes it read as expensive is the contrast in weight rather than the labels. A fluid bias-cut base under a stiff shouldered jacket creates the line people recognise. Swap either half for something in a matching weight, and the whole thing flattens out. So the pieces below are grouped as base, layer and finish, which is the order to shop them in.

The base should skim, not cling. A true bias cut in silk or satin falls closer to the body than a straight-cut slip, so size up if you are between sizes, and check the hem sits mid-calf or lower. A midi length is what makes the look read as considered instead of underdressed. Satin at a heavier weight, roughly 19 momme and up, hangs better and shows less than the thin stuff.

The layer needs a real shoulder. That means a structured blazer with a defined shoulder seam, not a soft cardigan-style jacket. Sized one to two up from your usual is the point: the sleeve should reach the base of your thumb and the shoulder seam should sit an inch or so past your own. Wool crepe and light double-face wool hold that line. Linen collapses.

On the finish, keep the bag small and slightly rigid. A compact shoulder bag or a structured mini reads correctly against a loose jacket, where a slouchy hobo disappears into it. For shoes, a pointed flat, a low square-toe mule or a clean loafer all work. Anything with an obvious platform pulls the proportion off.

For day, put the satin camisole under the blazer with straight trousers or denim and a flat shoe, and let the blazer stay open. This is the version that works for actual errands, and it is the one that translates furthest outside a photograph. Add a fine chain or a single cuff, and stop there, because the look depends on restraint in the jewellery.

For evening, take the same blazer over the slip dress and change nothing else except the shoe. A heeled sandal or a square-toe mule lifts it. Belt the blazer if you want a waist, but do not button it, since a closed blazer over a slip creates a bulk line at the hip.

In cold weather, the trick is a fine knit under the slip dress rather than over it: a slim long-sleeve crewneck or turtleneck beneath the straps keeps the neckline visible and stops the dress reading summery. Then a longer coat over the top instead of the blazer. Keep the coat unbuttoned so the column of the dress still shows, and keep tights sheer or skip them, because opaque tights fight the satin.

This is one of the few celebrity formulas that survives being copied, because it is genuinely a proportion trick rather than a shopping list. You do not need a specific brand for it to work. You need one fluid base, one jacket with a shoulder, and the discipline to stop adding.

Where people go wrong is buying the oversized blazer too big. Past about two sizes up it stops reading as borrowed and starts reading as ill-fitting, and no styling fixes that. The sleeve length is the tell: if it swallows your hand, it is too large.

If you take one thing away, take the weight contrast. Fluid under structured, small accessory, nothing else. That single rule gets you most of the way there in whatever you already own.