The underband around your body should be firm but comfortable. You should be able to run a finger under the band. If it is too tight, you need a larger size or wider hook placement. Make sure you fasten the bra on the first hook when first fitting.
The back and front of the bra should be parallel to the waistline or slightly lower in the back. The lower edge of the band should anchor between the shoulder blades. The lower the back fits, the better the support.
The centre gusset should lie flat against the breast bone, not bridge away from the body. If it doesn't, your cup size could be too small, or the panel could be too small to accommodate the separation between your breasts.
Underwires should lie flat against your rib cage, enclosing your breasts without rubbing or digging into the soft tissue of your armpit. If they bulge, consider a larger cup size.
The cup should have a smooth outline, without bulging at the top or the sides. The cups should not pucker or wrinkle.
The bra should support the weight of your breasts mainly with the underband. The cup and shoulder straps will share the remaining amount of support.
Check your silhouette. The bust should be midway between the top of the shoulder and the bend of the elbow, to create a silhouette of balanced proportions.
The straps should be adjusted to support comfortably, without pressure or cutting into the shoulder.