For the best comparison, lay a similar garment you already own flat and measure as outlined below. All garment measurements are taken flat.

Shoulder - Measure straight across from one shoulder seam to the other.
Bust - Measure straight across the garment from underarm to underarm, then double the measurement.
Waist - Measure straight across the garment at the waist, then double the measurement.
Hip - Measure straight across the garment at the widest point of the hip, then double the measurement.
Hem - Measure straight across the bottom edge of the garment, then double the measurement.
Length - Measure from the highest point of the garment down to the hem.
Sleeve Length - Measure from the shoulder seam to the end of the sleeve.
Rise - Measure from the top of the front waistband, through the crotch, to the top of the back waistband.