From the manufacturer

BRITISH LINGERIE, DESIGNED FOR YOU

WIRES

Front wires should sit flat at the breastbone between your breasts

UNDERBAND

Is it horizontal across your back? The underband should be snug, with enough room to fit two fingers underneath

CUPS

Your breasts should be fully enveloped in each cup, and there should be a smooth line where the cup ends

STRAPS

Straps should feel snug but not tight. If you loosen them, the bra should stay in the same place, since most of the support comes from the underband